ab initio study of the reaction of cho+ with h2o and nh3

Ab Initio Study of the Reaction of CHO+

with H O and NH2 3

´ ´ ´R. LOPEZ, E. DEL RIO, M. I. MENENDEZ, T. L. SORDODepartamento de Quımica Fısica y Analıtica, Facultad de Quımica, Universidad de Oviedo,´ ´ ´ ´CrJulian Claverıa, 8, 33006 Oviedo, Principado de Asturias, Spain´ ´

Received 10 March 1999; accepted 7 May 1999

Ž . Ž . Ž .ABSTRACT: An MP4 full,SDTQ r6-311qqG d,p rrMP2 full r6-311qqŽ .G d,p ab initio study was performed of the reactions of formyl and isoformyl

cations with H O and NH , which play an important role in flame and2 3interstellar chemistries. Two different confluent channels were located leadingto CO q H OqrNHq. The first one corresponds to the approach of the neutral3 4molecule to the carbon atom of the cations. The second one leads to the directproton transfer from the cations to the neutrals. At 900 K the separate productsCO q H OqrNHq are the most stable species along the Gibbs energy profiles3 4for the processes. For the reaction with H O the reaction channel leading to2

Ž .q Ž .HC OH protonated formic acid is disfavored with respect to the two2

CO q H Oq channels in agreement with the experimental evidence that H Oq3 3

is the major ion observed in hydrocarbon flames. According to our calculations,NHqq H O are considerably more stable in Gibbs energy than NH q H Oq;4 2 3 3

NHq will predominate in the reaction zone when ammonia is added to CH q4 4Ar diffusion flame, as experimentally observed. At 100 K the most stablestructures are the intermediate complexes CO . . . HOHqrHNHq. Particularly2 3the CO . . . HOHq complex has a lifetime large enough to be detected and,2therefore, could play a certain role in interstellar chemistry. Q 1999 John Wiley& Sons, Inc. J Comput Chem 20: 1432]1443, 1999

Keywords: ab initio calculations; hydrocarbon flames; interstellar chemistry;reaction channels; thermodynamic analysis

Correspondence to: T. L. Sordo; e-mail: [email protected]

Ž .Contractrgrant sponsor: DGUCYT Spain ; contractrgrantnumber: PB94-1314-C03-01

( )Journal of Computational Chemistry, Vol. 20, No. 13, 1432]1443 1999Q 1999 John Wiley & Sons, Inc. CCC 0192-8651 / 99 / 131432-12

REACTION OF CHO+ WITH H O AND NH2 3

Introduction

Ž q.he reactivity of formyl HCO and isoformylT Ž q. 1COH cations is important in flames andplasmas,2 and plays a central role in the chemistryof interstellar clouds.3

The formyl cation is always observed in hydro-carbon flames, and generates many of the ionspresent in these systems. In this high density envi-ronment HCOq undergoes rapid proton transferwith H O to produce H Oq, the major ion ob-2 3served in hydrocarbon flames. This positiveion]molecule reaction, which plays a role in hy-drocarbon flame ionization, has an enthalpy ofreaction as calculated from thermochemical data ofy23 kcalrmol.4a, b A second reaction channel fromHCOq q H O to give CH Oq has been also2 3 2reported.4 The experimental heat of reactionfor this process has been reported to bey40 kcalrmol4c and y43 kcalrmol4a, assumingthat the CH Oq species corresponds to protonated3 2formic acid. On the other hand, it has been experi-mentally observed that when ammonia is added toa CH q Ar diffusion flame5 NHq predominates4 4in the reaction zone. Under these circumstancesthe proton transfer between HCOq and NH can3play an important role as a source of NHq.4

It has been suggested that formation of formyland isoformyl cations in interstellar space occursmainly via the reaction of Hq with CO. The3most stable isomer, HCOq, is predominantly pro-duced.6, 7 Nevertheless, the generation of COHq insubstantial proportions would have important con-sequences with regard to the modeling of thechemistry in interstellar clouds.6 The reactions be-tween formyl and isoformyl cations with water toproduce H Oqq CO are important in this inter-3stellar chemistry. Given that NH has been de-3

Žtected in interstellar clouds although it is unlikely.to be present in large concentrations , the analo-

gous reactions for the proton transfer betweenformyl and isoformyl cations and ammonia mustalso play a certain role in interstellar chemistry.

Previous high level ab initio calculations havepredicted that HCOq and COHq are minimumenergy linear structures, the former being the moststable one by about 37.7 kcalrmol.8 ] 10 The experi-mental value is about 38.2 kcalrmol.11 The activa-tion energy for the isomerization reaction HCOqªCOHq is theoretically predicted to be about 72.6kcalrmol.8, 9 This energy barrier is substantially

lower than the dissociation energy, and therefore,a significant number of ions may be able to un-dergo such rearrangement.12 Very recently, theresults of the catalytic action of small neutralmolecules attached to the H atom in the rearrange-ment of HCOq into COHq have been reported.9, 13

In general, the interaction with a neutral moleculeleads to a significant lowering of the barrier for therearrangement. When the proton affinity of theneutral molecule lies above that of CO at carbon,as in the case of H O and NH , there is no suc-2 3cessful catalysis of the rearrangement from HCOq

to COHq because the proton is preferentiallytransferred to the neutral molecule rather thanmigrating to the O atom.

In the present work we performed an ab initiostudy of the mechanism of the following reactions:

q q Ž .HCO q H O ª H O q CO 12 3

q q Ž .COH q H O ª H O q CO 22 3

q q Ž .HCO q NH ª NH q CO 33 4

and

q q Ž .COH q NH ª NH q CO 43 4

in order to get a general understanding of theirrole in the chemistry of flames and interstellarclouds.

Methods

Ab initio calculations were performed with theGAUSSIAN 94 series of programs.14 Stable specieswere fully optimized, and transition statesŽ . 15TSs located using Schlegel’s algorithm at the

Ž . Ž . 16MP2 full r6-311 q q G d,p theory level. Allthe critical points were further characterized, and

Ž .the zero-point vibrational energies ZPVEs wereevaluated by analytical computations of harmonic

Ž .vibrational frequencies at the MP2 full r6-311qqŽ .G d,p level. Single-point calculations on the

Ž . Ž .MP2 full r6-311qqG d,p geometries were alsoŽ .carried out at the MP4 full,SDTQ r6-311q q

Ž .G d,p level.Ž . Ž . Ž .MP2 full r6-311 q q G d,p or MP2 full r6-

Ž . Ž .31G d Intrinsic Reaction Coordinate IRC calcula-tions starting at each saddle point verified the twominima connected by that TS using the Gonzalezand Schlegel method17 implemented in GAUSS-IAN 94.

D H, DS and DG values were also calculated toobtain results more readily comparable with ex-

JOURNAL OF COMPUTATIONAL CHEMISTRY 1433

´LOPEZ ET AL.

periment within the ideal gas, rigid rotor, andharmonic oscillator approximations.18 A pressureof 1 atm and 100, 298.15, and 900 K of temperaturewere assumed in the calculations.

Atomic charges were determined using a Natu-Ž . 19ral Bond Orbital NBO analysis. Ab initio wave

functions were analyzed by means of a theoreticalmethod developed by Fukui’s group20 based on

Ž .the expansion of the molecular orbitals MOs of acomplex A-B in terms of those of its fragments. Aconfiguration analysis is performed by writing theMO wave function of the combined system interms of various electronic configurations

Ž .c s C c q C C 5Ýo o q qq

Ž .where c zero configuration, AB is the state inowhich neither electron transfer nor electron excita-tion takes place, and c stands for monotrans-q

Ž q y y q.ferred configurations A B and A B , monoex-Ž .cited configurations A*B and AB* , and so on.

The analysis of the wave function was performedusing the ANACAL program.21

Results and Discussion

We present first the results obtained for theproton transfer from HCOq and COHq to H O2and then the corresponding to the proton transferto NH . Unless otherwise indicated, we will3

Ž .present in the text MP4 full,SDTQ r6-311qqŽ . Ž . Ž .G d,p rrMP2 full r6-311qqG d,p energies in-

cluding the MP2 ZPVE correction.

REACTIONS CHO++ H O ª CO + H O+2 3

Two different reaction channels appear for theattack of H O on both HCOq and COHq. The first2reaction channel corresponds to the interaction be-tween the oxygen atom of water and the carbonatom of the cations. The second one corresponds tothe direct transfer of a proton from the cations toH O. Figure 1 displays the optimized geometry of2the critical structures located along the reactioncoordinate for reactions 1 and 2. Table I collects therelative electronic energies of those structures.

Let us analyze first the results corresponding toŽthe reaction of the formyl cation with H O reac-2

.tion 1 . The first critical structure located along thefirst reaction channel is an intermolecular complexbetween HCOq and H O, MA1, which is220.8 kcalrmol under the reactants. According to an

NBO analysis, the net charge transfer from H O to2the formyl cation, which has lost its linearity, is0.05 e. This intermolecular complex can evolvethrough a TS, TSA12, with an energy barrier of0.3 kcalrmol to give a minimum structure, MA2,which is 37.0 kcalrmol more stable than reactantsŽ 9.the corresponding G2** value is 36.8 kcalrmol .Two main geometrical changes are observed whengoing from MA1 to TSA12: the oxygen atom of thewater molecule gets closer to the H atom in HCOq,and this reactant recovers its linearity. The netcharge transfer from H O at this TS is 0.03 e. At2MA2, which has C symmetry, HCOq has trans-sferred the proton to the water molecule. MA2 canisomerize through a TS for inversion in the H Oq

3moiety, TSA22, of C symmetry with an energy2vbarrier of 0.4 kcalrmol. Finally, MA2 can alsoproceed directly to the products CO q H Oq,3

Žwhich are 22.6 kcalrmol under reactants the cor-9.responding G2** value is 22.5 kcalrmol in agree-

ment with the greater proton affinity of H O than2that of CO at C. The energy profile for the secondchannel corresponding to the direct proton transferdecreases monotonously without encountering anyenergy barrier to form MA2.

Concerning reaction 2, the first reaction channelleads to a minimum structure, MA3, of C symme-1try, which is 19.4 kcalrmol more stable than reac-tants. In MA3, the oxygen atom of the watermolecule is bound to the carbon atom of the iso-formyl cation forming an angle of 101.48 with theC—O bond. The H atom of the COHq moietyforms an angle of 109.38 with the same bond, andpresents an outward orientation. The net chargetransfer from the H O moiety to the COHq frag-2ment is 0.38 e. MA3 is connected with a minimumstructure, MA4, 71.7 kcalrmol more stable than

Žreactants the corresponding G2** value is 68.99.kcalrmol , through a TS, TSA34, with an energy

barrier of 3.7 kcalrmol. At TSA34 the H O2˚molecule has moved to a distance of 2.465 A away

from the COHq fragment, which has recovered itslinearity. At this TS the net charge transfer fromH O is only 0.02 e, given that the distance be-2tween both fragments is relative large to facilitatethe rotation of the isoformyl moiety. At MA4 theisoformyl cation has transferred a proton to thewater molecule determining a C symmetry. MA4scan isomerize through a TS for inversion in theH Oq moiety, TSA44, of C symmetry with an3 2venergy barrier of 0.9 kcalrmol. MA4 can alsoevolve directly to the products, CO q H Oq,3which are 64.6 kcalrmol more stable than reac-tants. The second reaction channel for the direct

VOL. 20, NO. 131434


( ) ( )FIGURE 1. MP2 full / 6-311++G d,p optimized geometries of the chemically important structures located for thereactions of formyl and isoformyl cations with H O. Bond lengths are given in angstroms and bond angles in degrees.2

proton transfer gives MA4 along a monotonouslydecreasing energy profile without encounteringany energy barrier.

For both processes 1 and 2 the approach of theoxygen atom of water to the carbon atom of thecations leads to the formation of minimum energystructures through the interaction between the

Ž .Next HOMO NHOMO of water and the LUMOŽ .of the cation see Table II . However this interac-

tion presents a smaller relative weight for MA1,where the HCOq]H O system is mainly stabilized2

by electrostatic interaction between the oxygenatom of water and the carbon atom, which is themost positively charged atom in the formyl cation,both fragments being quite separate from eachother. In effect, the total bond order between thefragments is only 0.051 at MA1. The search for aminimum energy structure presenting a stronger

bonding between the O atom of water and the Catom of the formyl cation failed. No intermediatewith the H atom oriented inwards was located forthe COHq]H O system.2

Ž .Three TSs TS00, TSA13, and TSA24 have beenfound directly connecting the energy profiles forreactions 1 and 2. TS00 connects separate reactantsof reactions 1 and 2 and corresponds to the isomer-ization of HCOq to COHq. According to our calcu-lations, TS00 is 76.1 kcalrmol less stable thanHCOq, which in turn is 42.0 kcalrmol more stablethan COHq. These results are comparable with

Ž .those obtained previously at MP2 full r6-311qqŽ . Ž .G 3df,3pd 83.3 and 44.2 kcalrmol and

Ž .Ž . Ž .QCISD T full r6-311qqG 2df,p rrB3LYPr6-311Ž . Ž . 13qG d,p 73.9 and 38.8 kcalrmol levels. TSA13

is 55.1 kcalrmol above HCOqq H O, and con-2

nects MA1 with MA3. This is a saddle point for


´LOPEZ ET AL.

TAB

LEI.

()

()

()

()

()

Rel

ativ

eM

P2

full

/// //6-3

11+

+G

d,p

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rgie

sD

E,Z

ero

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intV

ibra

tiona

lEne

rgy

Co

rrec

tions

ZPV

E,r

elat

ive

MP

4fu

ll,S

DTQ

/// //6-3

11+

+e

lec

()

()

()

()

()

()

()

Gd

,p/// ///// //

MP

2fu

ll/// //6

-311

++

Gd

,pE

nerg

ies

DE

,Ent

hal

pie

sD

H,E

ntro

py

Co

rrec

tions

yTD

S,a

ndG

ibb

s’Fr

eeE

nerg

ies

DG

,in

ele

ca

kcal

/// //mo

l,o

fCh

emic

ally

Imp

ort

antS

truc

ture

sfo

rth

eR

eact

ion

ofF

orm

ylan

dIs

ofo

rmyl

Cat

ions

with

HO

.2

T=

100

KT

=29

8.15

KT

=90

0K

bc

Str

uctu

reD

EZP

VE

DE

DH

yTD

SD

GD

Hy

TDS

DG

DH

yTD

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Ge

lec

ele

c

+H

O+

HC

O0.

023

.82

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

2M

A1

y22

.525

.61

y22

.6y

21.3

2.2

y19

.1y

21.3

6.8

y14

.5y

19.9

18.1

y1.

8TS

A12

y22

.225

.41

y22

.1y

21.1

2.4

y18

.7y

21.6

8.1

y13

.5y

21.2

23.9

2.7

MA

2y

38.6

26.4

0y

39.6

y37

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5y

35.1

y37

.98.

0y

29.9

y37

.223

.2y

14.0

TSA

22y

37.2

25.1

6y

37.9

y37

.22.

6y

34.6

y37

.68.

6y

29.0

y37

.626

.3y

11.3

+H

O+

CO

y22

.125

.02

y23

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22.6

y0.

2y

22.8

y22

.6y

0.7

y23

.3y

23.1

y1.

2y

24.3

3H

O+

CO

H+

47.4

22.2

743

.542

.0y

0.02

42.0

42.4

y0.

741

.743

.1y

3.4

39.7

2M

A3

21.5

27.3

219

.121

.92.

624

.521

.29.

230

.421

.927

.048

.9TS

A34

30.4

23.6

526

.525

.82.

328

.126

.06.

632

.627

.117

.945

.0M

A4

y29

.826

.48

y32

.4y

30.2

2.3

y27

.9y

30.2

7.0

y23

.2y

29.3

19.6

y9.

7TS

A44

y28

.125

.36

y30

.4y

29.4

2.5

y26

.9y

29.5

7.6

y21

.9y

29.3

22.6

y6.

7H

O+

TS00

82.8

19.6

080

.376

.2y

0.4

75.8

76.3

y1.

375

.075

.5y

2.7

72.8

2TS

A13

57.8

22.7

856

.154

.42.

556

.954

.18.

262

.354

.923

.578

.4TS

A24

y23

.425

.67

y25

.2y

23.7

1.9

y21

.8y

23.7

5.7

y18

.0y

23.7

17.2

y6.

5

a(

)(

)D

H=

DE

+D

E+

DE

+D

nRT;

ther

mal

and

ZPV

Een

ergi

esw

ere

obta

ined

from

the

MP

2fu

ll/6

-311

++

Gd,

pfre

quen

cies

,w

hile

elec

tron

icco

ntrib

utio

nsw

ere

ele

cZ

PV

Eth

erm

al

()

()

deriv

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mth

eM

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full,

SD

TQ/6

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++

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psi

ngle

-poi

ntca

lcul

atio

ns.

b(

)(

)R

elat

ive

MP

2fu

ll/6

-311

++

Gd,

pen

ergi

es.

c(

)(

)(

)(

)R

elat

ive

MP

4fu

ll,S

DTQ

/6-3

11+

+G

d,p

//M

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full

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11+

+G

d,p

ener

gies

.

VOL. 20, NO. 131436


the migration in the cationic fragment of the Hatom from C to O. At TSA13, the H atom ishalfway this motion interacting simultaneouslywith C and O atoms. Comparing TSA13 with TS00we see that when a water molecule is attached tothe C atom the TS for the migration of a H atomfrom C to O becomes tighter with shorter H—C

Ž .and H—O distances see Fig. 1 . TSA13 is 21.0kcalrmol more stable than H O q TS00. Never-2theless, the presence of the water molecule doesnot seem to imply a greater electron transfer to theproton at the saddle point because the charge ofthe proton at TS00 and TSA13 is practically the

Ž .same q0.57, q0.59 . When going from MA1 toMA3 through TSA13 the C—OH distance contin-2

˚uously decreases from 2.261 to 1.528 A. The netcharge transfer increases from 0.05 e at MA1 to0.25 e at TSA13 and to 0.38 e at MA3. TSA24 is

q Ž23.3 kcalrmol more stable than HCO q H O the29.corresponding G2** value is 23.9 kcalrmol and

99.4 kcalrmol more stable than H O q TS00, and2

connects the ion-neutral complexes MA2 and MA4.At TSA24, the proton detached from the C atomand solvated with the water molecule is placedapproximately in front of the middle point of the C—O bond.

REACTIONS CHO++ NH ª CO + NH+3 4

Two different reaction channels also appear forthe attack of NH on both HCOq and COHq. The3

first reaction channel corresponds to the interac-tion between the nitrogen atom of ammonia andthe carbon atom of the cations. The second onecorresponds to the direct transfer of a proton toNH . Figure 2 displays the optimized geometry of3

the critical structures located along the reactioncoordinate for reactions 3 and 4. Table III collectsthe relative electronic energies of those structures.

( ) ( )FIGURE 2. MP2 full / 6-311++G d,p optimized geometries of the chemically important structures located for thereactions of formyl and isoformyl cations with NH . Bond lengths are given in angstroms and bond angles in degrees.3


´LOPEZ ET AL.

We shall first present the results correspondingto the proton transfer from the formyl cation to theammonia molecule. When ammonia approachesthe C atom of HCOq, a minimum energy structureof C symmetry, MB1, appears 43.0 kcalrmol un-sder reactants. In MB1, the nitrogen atom of ammo-nia is linked through its lone pair of electrons tothe positively charged carbon atom of the formylcation by a dative covalent bond with a bond

˚ qdistance of 1.568 A. The HCO fragment presentsan H—C—O angle of 130.98. The net charge trans-fer from ammonia is 0.58 e. MB1 can evolvethrough a TS of C symmetry, TSB12, to give asminimum energy structure of C symmetry, MB2.3vAt TSB12, which is 19.4 kcalrmol above MB1, the

˚Ž .nitrogen atom of ammonia is closer 2.149 A to thehydrogen atom of the formyl cation, which has

Žalmost completely recovered its linearity the.H—C—O angle is 176.88 . The net charge transfer

from ammonia is only 0.05 e. At MB2, which isŽ69.1 kcalrmol more stable than reactants the cor-

9.responding G2** value is 68.8 kcalrmol , a pro-ton has been transferred to the ammonia moleculeoriginating an OC . . . H-NHq complex with a C-3

˚proton distance of 2.073 A. Finally, MB2 can evolvedirectly to the products, CO q NHq, which are4

Ž62.3 kcalrmol more stable than reactants the cor-9.responding G2** value is 62.1 kcalrmol . The

energy profile for the second channel correspond-ing to the direct proton transfer from the cationdecreases monotonously without encountering anyenergy barrier to form MB2.

As for reaction 4, the approach of NH to COHq3

along the first reaction channel can lead to twodifferent minimum energy structures: MB3 of C1

symmetry and MB5 of C symmetry. MB3 is 32.4skcalrmol more stable than reactants, while MB5 is12.3 kcalrmol more stable than MB3. In MB3, thenitrogen atom is bound to the carbon atom of the

˚isoformyl cation with a bond length of 1.603 A,and the isoformyl moiety presents a C—O—Hangle of 121.98 with the hydrogen atom orientedinwards. This orientation determines a repulsionbetween that H and the H atom of the ammoniafragment that are separated by a distance of

˚2.236 A. In MB5, the nitrogen atom is also bound˚to the carbon atom with a bond length of 1.545 A

and the C—O—H angle is 108.98, with the hydro-gen atom oriented outwards. This orientationavoids the H—H repulsion present in MB3, thusallowing a shorter C—N bond length and an im-portant energy stabilization with respect to MB3.The net charge transfer from ammonia at MB3 and

Ž .MB5 is 0.48 and 0.54 e, respectively see Table II .MB3 and MB5 are connected each other throughTSB35 by rotation of the O—H bond about the C—O bond. TSB35 is 10.9 kcalrmol less stable thanMB3. At TSB35, the net charge transfer from am-monia is 0.38 e. MB3 is connected with a mini-mum energy structure MB4 through a TS, TSB34,with an energy barrier of 11.4 kcalrmol. At TSB34,the net charge transfer from ammonia is 0.06 e.The COHq moiety rotates about an axis perpen-dicular to the molecular plane and passing throughthe oxygen atom, to insert the hydrogen atom intothe C—N bond and give MB4, which is a CO . . . H—NHq complex 108.0 kcalrmol more stable than3

Žreactants the corresponding G2** value is 104.29.kcalrmol . MB4 can evolve directly to the prod-

ucts CO q NHq, which are 104.2 kcalrmol more4

TABLE II.( + +? )Coefficients of the Most Important Electronic Configurations A = HCO ///// COH , B = H O ///// NH , Most Important2 3

Changes in the Occupation Numbers of the Fragment MOs, Dn, Net Charge Transfer from H O ///// NH to2 3+ +HCO ///// COH , CT, and Total Mayer Bond Orders Between the Fragments.

MA1 MA3 MB1 MB3 MB5

AB 0.9776 0.5967 0.4768 0.5162 0.4397y +A B NHOMO-LUMO y0.1142 y0.2773

HOMO-LUMO 0.0260 0.0965 0.4232 y0.3859 y0.3949( )Dn HOMO B y0.01 y0.11 y0.69 y0.62 y0.71

( )NHOMO B y0.04 y0.36 y0.02 y0.03 y0.03( )LUMO A 0.06 0.50 0.71 0.66 0.74

CT B ª A 0.05 0.38 0.58 0.48 0.54Total Mayer bond order 0.051 0.588 0.639 0.578 0.622

VOL. 20, NO. 131438


TAB

LEII

I.(

)(

)(

)(

)(

)R

elat

ive

MP

2fu

ll/// //6

-311

++

Gd

,pE

nerg

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oin

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ratio

nalE

nerg

yC

orr

ectio

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ativ

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full,

SD

TQ/// //6

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++

ele

c(

)(

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d,p

/// ///// //M

P2

full

/// //6-3

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d,p

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rgie

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nth

alp

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DH

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rop

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orr

ectio

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,and

Gib

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Free

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rgie

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G,i

ne

lec

akc

al/// //m

ol,

ofC

hem

ical

lyIm

po

rtan

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res

for

the

Rea

ctio

no

fFo

rmyl

and

Iso

form

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H.

3

T=

100

KT

=29

8.15

KT

=90

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bc

Str

uctu

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DE

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SD

GD

Hy

TDS

DG

DH

yTD

SD

Ge

lec

ele

c

+N

H+

HC

O0.

032

.08

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

3M

B1

y47

.836

.86

y47

.8y

43.6

2.8

y40

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44.4

9.7

y34

.7y

44.6

29.8

y14

.8TS

B12

y24

.833

.32

y24

.8y

24.1

2.4

y21

.7y

24.4

7.7

y16

.7y

24.0

22.7

y1.

3M

B2

y71

.535

.87

y72

.9y

69.6

2.5

y67

.1y

69.6

7.5

y62

.1y

69.0

21.9

y47

.1+

NH

+C

Oy

62.9

34.5

4y

64.7

y62

.2y

0.1

y62

.3y

62.3

y0.

1y

62.4

y63

.41.

5y

61.9

4+

NH

+C

OH

47.4

30.5

343

.542

.0y

0.02

42.0

42.4

y0.

741

.743

.1y

3.4

39.7

3M

B3

8.4

35.9

25.

78.

92.

811

.78.

29.

717

.98.

628

.837

.4TS

B34

24.4

32.6

120

.420

.42.

522

.920

.18.

028

.120

.723

.243

.9M

B4

y67

.035

.46

y69

.4y

66.4

2.5

y63

.9y

66.1

7.1

y59

.0y

65.4

19.3

y46

.1TS

B35

22.3

33.1

019

.419

.92.

722

.619

.68.

528

.120

.124

.945

.0M

B5

y5.

536

.93

y7.

6y

3.4

2.8

y0.

6y

4.2

9.9

5.7

y4.

230

.226

.0N

H+

TS00

82.8

27.8

680

.376

.2y

0.4

75.8

76.3

y1.

375

.075

.5y

2.7

72.8

3TS

B15

32.2

32.7

831

.031

.12.

833

.930

.49.

640

.030

.329

.559

.8TS

B24

y63

.434

.98

y65

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62.7

1.7

y61

.0y

62.6

5.1

y57

.5y

63.1

16.0

y47

.1

a(

)(

)D

H=

DE

+D

E+

DE

+D

nRT;

Ther

mal

and

ZPV

Een

ergi

esw

ere

obta

ined

from

the

MP

2fu

ll/6

-311

++

Gd,

pfre

quen

cies

,whi

leel

ectr

onic

cont

ribut

ions

wer

ee

lec

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the

rma

l(

)(

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rived

from

the

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d,p

sing

le-p

oint

calc

ulat

ions

.b

()

()

Rel

ativ

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P2

full

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d,p

ener

gies

.c

()

()

()

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Rel

ativ

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Gd,

pen

ergi

es.


´LOPEZ ET AL.

stable than reactants. The second reaction channelfor direct proton transfer from the cation givesMB4.

For the reaction of formyl and isoformyl cationswith ammonia the interaction between the nitro-gen atom and the carbon atom takes place througha charge transfer from the HOMO of ammonia to

Ž .the LUMO of the cation see Table II . Given thatammonia is a stronger base than water, the energygap between the interacting fragment MOs issmaller now than in the case of reactions 1 and 2.As a consequence, the minimum energy structuresformed and, in general, all the energy profile cor-responding to the reaction between ammonia andformyl and isoformyl cations are considerably morestable than before. In MB1, MB3, and MB5 the two

˚fragments are separated by 1.55]1.60 A, and theŽtotal bond order between them is 0.55]0.65 see

.Table II . The energy barriers connecting MB1 withMB2, and MB3 with MB4 are greater, respectively,than those connecting MA1 with MA2, and MA3with MA4.

In addition to TS00, which connects separatereactants of reactions 3 and 4, two other TSs,TSB15 and TSB24, have been found connectingthe energy profiles for these reactions. TSB15 con-nects MB1 with MB5, and corresponds to themigration of the H atom in the cationic moietyfrom C to O. TSB15 is 31.7 kcalrmol less stablethan HCOqq NH , and 44.4 kcalrmol more stable3than NH q TS00. At this TS the net charge trans-3fer from ammonia is 0.57 e. Comparing TSB15with TS00, we see that when an ammonia moleculeis attached to the C atom the TS for the migrationof an H atom from C to O becomes even tighterthan TSA13 with shorter H—C and H—O dis-

Ž .tances see Fig. 2 . Nevertheless, the presence ofthe ammonia does not cause a greater electrontransfer to the proton at the saddle point becausethe charge of the proton at TS00 and TSB15 is very

Ž .similar q0.57, q0.60 despite the negative chargeof the CO fragment. TSB24 is 62.4 kcalrmol more

q Žstable than HCO q NH the corresponding G2**39.value is 62.4 kcalrmol and 138.5 kcalrmol more

stable than NH q TS00, and connects the two3complexes MB2 and MB4. In TSB24, one of thehydrogen atoms of NHq is placed in front of the4C—O bond with C—H and O—H distances of

˚ Ž .2.749 and 2.638 A, respectively see Fig. 2 .According to an NBO analysis in both formyl

and isoformyl cations C is the most positivelyŽ .charged atom q0.92, and q1.0 e, respectively .

Nevertheless, the H atom has a positive charge ofq0.31 and q0.69 e, respectively. Therefore, al-

though for all four processes direct proton transfercan take place without an energy barrier, the ap-proach of the neutral molecule to the C atom of thecation could also play a certain role.

THERMODYNAMIC ANALYSIS

To discuss the role of reactions 1]4 in the chem-istry of combustion and interstellar clouds we per-formed a thermodynamic analysis of our theoreti-cal results at three different temperatures: 100,298.15, and 900 K.

Figures 3 and 4 display the Gibbs energy pro-files for reactions CHOqq H O ª CO q H Oq

2 3

FIGURE 3. Gibbs energy profiles for the possible( )reaction channels continuous and dashed lines

corresponding to the reaction of formyl and isoformylcations with H O, and connections between them at2( ) ( )a 900 K and b 100 K.

VOL. 20, NO. 131440


FIGURE 4. Gibbs energy profiles for the possible( )reaction channels continuous and dashed lines

corresponding to the reaction of formyl and isoformylcations with NH , and connections between them at3( ) ( )a 900 K and b 100 K.

and CHOqq NH ª CO q NHq, respectively, at3 4900 and 100 K. Tables I and III collect D H, yTDSand DG for these processes at 100, 298.15, and900 K.

The D H value predicted by our calculations forthe reaction HCOqq H O ª CO q H Oq, y22.62 3

kcalrmol, is in good agreement with the experi-mental value of y23 kcalrmol.4a, b For the reaction

q Ž .q ŽHCO q H O ª HC OH protonated formic2 2.acid we obtained a D H of y33.8 kcalrmol to

compare with the reported experimental values ofy40 kcalrmol4c and y43 kcalrmol. The proton

Žaffinity difference between NH and H O the DH3 2q q.for the reaction NH q H O ª NH q H O4 2 3 3

Ž .obtained by us is 39.7 kcalrmol 298.15 K ingood accordance with the reported experimentalvalues: 34 " 5 kcalrmol,4b 37.5 kcalrmol,11b and38.5 kcalrmol.22

From Table I we see that, owing basically to theentropic contribution at 900 K, the Gibbs energyprofiles for both reactions 1 and 2 are about 17]27kcalrmol less stable with respect to HCOqq H O2than the electronic energy profiles except for TS00and products for which yTDS is y2.7 kcalrmoland y1.2 kcalrmol, respectively. This determinesthat products are the most stable species along the

Ž .whole Gibbs energy profile see Fig. 3a . Giventhat protonated formic acid is 21.5 kcalrmol lessstable than CO q H Oq in Gibbs energy, our cal-3culations render at this temperature the reactionchannel leading to protonated formic acid disfa-vored compared with that leading to CO q H Oq

3in agreement with experimental evidence thatH Oq is the major ion observed in hydrocarbon3flames. Analogously, Table III shows that at 900 Kthe entropic contribution makes the Gibbs energyprofiles for reactions 3 and 4 less stable with re-spect to HCOqq NH in general than the elec-3tronic energy profiles. MB1, MB3, MB5, and TSB15present a yTDS value of about 30 kcalrmol,whereas the remaining critical structures have ayTDS value of 16]25 kcalrmol approximately,

Ž .except for TS00 and products 1.5 kcalrmol . Asa result, the products CO q NHq are the most4stable species along the Gibbs energy profile for

Ž . Ž .reactions 3 and 4 see Fig. 4a . DG 900 Kfor the reaction NHqq H O ª NH q H Oq is4 2 3 337.6 kcalrmol, in agreement with the experimentalobservation that when ammonia is added to aCH q Ar diffusion flame5 NHq predominates4 4in the reaction zone.

At 100 K as the yTDS term is about nine timessmaller in absolute value than at 900 K, the sepa-rate products are not the most stable species alongthe Gibbs energy profiles for reactions 1]4. Ineffect, Figures 3b and 4b show that at 100 K foreach of these reactions there is an intermediatecorresponding to the interaction between a COmoiety and a protonated base fragment that is


´LOPEZ ET AL.

more stable than products. Thus, MB4 and MB2are 1.6 and 4.8 kcalrmol more stable than CO qNHq, respectively, and MA4 and MA2 are 5.1 and412.3 kcalrmol more stable than CO q H Oq, re-3spectively, in Gibbs energy. Furthermore, our cal-culations predict that MA2 could be detected ininterstellar clouds and play a certain role in inter-stellar chemistry given that this intermediate has alifetime of 3.67 ? 1014 s, much greater than that ofthe other three above-mentioned intermediates,which present a lifetime shorter than 7 10y2 s.23

Ž .In summary, the MP4 full,SDTQ r6-311qqŽ . Ž . Ž .G d,p rrMP2 full r6-311qqG d,p study of the

reaction between formyl and isoformyl cations withwater and ammonia shows that these processesproceed through two different channels. The firstone corresponds to the attack of the central atomof the neutral to the carbon atom of the cation,giving rise to a first intermediate. This evolves togive a second intermediate corresponding to theinteraction of a CO molecule with the protonatedneutral, which in turn, leads to the separate prod-ucts. Along the second reaction channel the directprotonation of the neutral takes place to producethe second intermediate mentioned above. At900 K, the separate products, CO q H OqrNHq,3 4are the most stable species along the Gibbs energyprofiles for the studied processes. Our theoretical

Ž .results y22.6 kcalrmol reproduce quite well theD H for the reaction between formyl cation andwater to give CO q H Oq evaluated from experi-3

Ž .mental data y23 kcalrmol . For the reactionchannel giving protonated formic acid, our calcu-lations predict a D H of reaction of y33.8 kcalrmolto compare with the experimental value of y40kcalrmol. At 900 K, our calculations render thereaction channel, leading to protonated formic aciddisfavored compared with that leading to CO qH Oq, in agreement with the experimental evi-3dence that H Oq is the major ion observed in3hydrocarbon flames. According to our calculations,

Ž . qthe DG 900 K for the process NH q H O ª4 2NH q H Oq is 37.6 kcalrmol in agreement with3 3the experimental observation that when ammoniais added to a CH q Ar diffusion flame NHq

4 4predominates in the reaction zone. Unlike at 900 K,at 100 K the most stable structure located along theGibbs energy profiles is the intermediate complexCO . . . HOHqrHNHq corresponding to the cap-2 3ture of a proton by the neutral. Particularly theCO . . . HOHq complex is 12.3 kcalrmol more sta-2ble than CO q H Oq, and has a lifetime of 3.67 ?31014 s and, consequently, according to our results,

it could be a detectable species playing a certainrole in interstellar chemistry.

Acknowledgments

Ž .The authors are grateful to DGICYT Spain forŽ .financial support PB94-1314-C03-01 . E. del R. also

thanks to the DGICYT for a grant.

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VOL. 20, NO. 131442


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23. The mean lives of MA2 and MA4 intermediates wereestimated as t s 1rk , where k is the kinetic constantdiss dissfor the fragmentation of the MA2 and MA4 structures toCO q H O. k was computed using the conventional3 diss

Ž . Ž a .transition state theory: k s kTrh exp yDG rRT ,disswhere DGa is the Gibbs energy barrier for the fragmenta-tion as shown in Table I.


ab initio study of the reaction of cho+ with h2o and nh3

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