Gas phase studies of metal complexes, isomeric carbanions and distonic radical anions under soft ionization mass

spectral conditions

VIVA OF THE THESIS Presented to

OSMANIA UNIVERSITY

BY

M. Kiran Kumar

(Mentor: Dr. M. Vairamani)National Centre for Mass Spectrometry (NCMS)Indian Institute of Chemical Technology (IICT)

CHAPTER 1 Co-ordination chemistry of [Cr(III) Salen] compounds under electrospray ionization conditions

CHAPTER 2 The effect of spacer chain length on ion binding to -diamines and diols: contrasting ordering for H+ and alkali metal ion affinities

CHAPTER 3 Generation of regiospecific carbanions under electrospray ionization conditions and characterization by ion-molecule reactions with carbon dioxide

CHAPTER 4 Generation of distonic dehydrophenoxide radical anions under electrospray and atmospheric pressure chemical ionization conditions

Chapter 1: Co-ordination chemistry of [Cr(III) Salen] compounds under electrospray ionization conditions

• Characterization of the metal complexes and to identify the crucial intermediates in metal-mediated reactions in order to understand the nature and reactivity of metal complexes and their reaction pathways.

• The study of metal complex systems using MS (i.e., in the gas phase) is a rapidly expanding field of research

• Knowledge of the gas-phase structures of metal complexes is important for analytical applications, as evidenced by several reviews.

• Interest in Salen type complexes intensified in 1990 when the groups of Jacobsen and Katsuki discovered the enantioselective epoxidation of unfunctionalised alkenes using chiral MnIII(Salen) complexes as catalysts

► ESI has proven to be a soft ionization method that keeps intact any weakly bound ligands in a complex ion.

► Axial positions of [M(salen)] are much important will enhance the yield of the epoxidation reaction.

► Recently the axial interactions with DNA, nucleotides and nucleosides were studied by this technique.

► We present here axial positions study of [Cr(Salen)] complex using a primary amine and a series of diamines as ligands.

CrN N

O OIII

. PF6

L

L► However, there are few reports on the EI

studies on a few metal-Salen complexes.

► Electrospray ionization (ESI) is a method to study ionic complexes.

► Epoxidation of olefins in solution was also confirmed in the gas phase by applying ESI method to [Mn(Salen)] complexes.

• The positive ion ESI mass spectrum of [CrIII(Salen)]+ complex in acetonitrile (ACN) shows M+, [M(ACN)]+, [M(ACN)2 ]+ ions.

The ESI mass spectra of [Cr(Salen)]PF6

in acetonitrile at different cone voltages

10 eV

20 eV

30 eV

CrIII(Salen)Solvent-ACN

ESI

Low cone

High cone

Abundant ion

[CrIII(Salen)(ACN)2]+

[CrIII(Salen)]+

• Significant abundance differences with varying the cone voltage.

CONE Fragmentation

Capillary Voltage : 3-5 kVCone: 5-100 V

++++

+

+++

++++ ++

MH+

S

SS

S

• In the presence of propylamine (PA) clearly demonstrates the displacement of solvent molecules present in the axial positions by the stronger ligand.

CrIII(Salen)

+ PA

ESI

Low cone

High cone

[CrIII(Salen)(PA)2]+

[CrIII(Salen)(PA)(ACN)]+

Abundant ions

[CrIII(Salen)(PA)2]+

High cone [CrIII(Salen)]+

[CrIII(Salen)PA]+

Surrounding solvent ACN[CrIII(Salen)(ACN)]+

[CrIII(Salen)(ACN)2]+

[CrIII(Salen)PA (ACN)]+

A series of primary diamines (DA) studied to see the effect of chain length and bidentate nature on the occupation of the axial positions of CrIII(Salen)]+.

H2N-(CH2)n-NH2 n = 2-8.

10 eV

20 eV

30 eV

+ H2N (CH2)n NH2

CrN N

O OIII

. PF6ESI III

N N

O OCr

NH2

(CH2)nNH2

+

1. Source experiments2. Ion-Molecule reactions3. CID Experiments

Depends on the binding strength of the Diamine the abundances of surrounded solvent adducts will be varied.

1. Source experiments

[CrIII(Salen)(ACN)]+

[CrIII(Salen)(ACN)2]+

Surrounding solvent ACN

[CrIII(Salen)] [CrIII(Salen)(DA)]+

High cone

Propane Diamine

Hexane Diamine

The order of bidentate nature of the diamines towords [CrIII(Salen)]+ can be given as

H2N-(CH2)n-NH2 n = 2-8 (1-7). 3 >2 >4 >5 8 >7 >6.

IonRelative abundance (%)

1 2 3 4 5 6 7

[CrIII(Salen)]+

m/z 3182.9 4.3 10 19 44 34 40

[CrIII(Salen)(ACN)]+

m/z 3595.8 7.9 20 37 65 61 66

[CrIII(Salen)(ACN)2]+

m/z 4004.4 7.1 18 35 56 54 48

[CrIII(Salen)(DA)]+ 100 100 100 100 100 100 100

[CrIII(Salen)(DA)(ACN)]+ - - 2.9 5.1 5.1 0.7 0.3

[CrIII(Salen)(DA)2]+ - - 1.5 4.4 5.1 8.7 13

Table 1: Positive ion ESI mass spectra (cone voltage 30 V) of mixtures of [CrIII(Salen)]+ (as the PF6

- salt) with diamines (DA) ligands (1-7) in acetonitrile (ACN) solvent.

Ligand-Pickup Experiments:

The ion of interest can be selected by MS1 and allowed to undergo ion-molecule reactions with the ligand of interest.

Empty axial positions of [CrIII(Salen)]+ ion are occupied by any ligand in collision cell.

The displacement of weaker ligands in the axial positions by stronger ligands was also observed this experiments.

Schematic diagram of ESI Mass Spectrometer

ligand-pickup experiments by selecting [Cr(Salen)(PA)]+ and [Cr(Salen)(hexd)]+ ions using acetonitrile as the collision gas.

Selected ions

in MS1

Resulted

ions in MS2

Cr

N

N

PA

Cr

Ligand-Pickup Experiments:

All the diamines(DA) are bidentate in nature with [CrIII (Salen)]+ at its axial positions.

From these experiments diamines 6,7 and 8 shown to be week in bidentate (not mono dentate) nature than the other diamines.

[M(DA) L]+

[M(DA)]+

MS2

[M(DA)]+

MS1

L

Col.Cell

Mono

Bi

DA =

Fig: The plot of Pc/Pd ratios ([CrIII(Salen)(DA)]+/ [CrIII(Salen)]+ obtained at collision energies of 10, 12 and 14 eV from CID of [CrIII(Salen)(DA)]+ ions for ligands (Diamines) 1-7.

[Cr(Salen)(DA)]+

MS1

[Cr(Salen)]+

MS2

Ar

Col.Cell

CID Experiments: (MS/MS)

CID Experiments: (MS/MS)

[Cr(Salen)(DA)]+

MS1

The order of stabilities of [CrIII(Salen)(DA)]+ complexes for diamines 2-8 can be given as 3 >2 >4 >5 8 >7 >6 from Pc/Pd ratios.

[Cr(Salen)]+

MS2

Ar

Col.Cell

Pc/Pd = Relative strength of the Diamines

1. Source experiments2. Ion-Molecule reactions3. CID Experiments

The relative binding strength of the Diamines towards [Cr(Salen)]+

H2N-(CH2)n-NH2 n = 2-8.

3 >2 >4 >5 8 >7 >6

Imp: Understanding the Metal complexes under MS conditions

• Knowledge of accurate H+ and M+ ion binding interactions in poly-functional macromolecules is an essential step in understanding the biophysical processes.

• The estimation of thermo chemical properties to the mono-functional molecules is very much straight forward, whereas evaluation to the molecules with two or more functional groups and chain length are particularly interesting i.e. bi-functional/ poly-functional case, because there will be an internal hydrogen bonding between the functional groups.

• Protonation of -diamines has been also extensively studied using other mass spectrometric methods and computational techniques.

Chapter 2: The effect of spacer chain length on ion binding to Chapter 2: The effect of spacer chain length on ion binding to bidentate ligands: Contrasting ordering for Hbidentate ligands: Contrasting ordering for H++ and Alkali Metal ion and Alkali Metal ion

affinitiesaffinities

Bidentate Ligands: Diamines and Diols

A (CH2)n AH+

A

(CH2)n

A

H+

A = NH2, OH

The Kinetic Method

• The method has been successfully used for the determination of proton affinities, gas phase acidities, metal, chloride ion affinities, etc.

• This method was developed by Cooks and co-workers, is an effective method for estimating the relative binding energies of two similar bases that bind to a central ion, typically a proton/metal ion.

• Basically, the kinetic method consists in relating the ratio of the peak intensities associated with two competitive dissociation channels (heterodimer) to a difference in thermo-chemical properties of the corresponding products.

[L1- - -M+- - -L2] L1 + L2M+

L2 + L1M+ (rate constant = k2) (3)

(rate constant = k1) (2)

ln([L2M+]/[L1M+]) (HML2 - HML1)/RTeff RTeff (6)~~ ~~

Measured ln(ILi+

-DA2/ILi+

-DA1) values for Li+-bound heterodimers of diamines (1–7). The data presented under the heading ln(ILi

+-DA2/ILi

+-DA2) are average cumulative values expressed relative to ethylene diamine (1). The numbers

given in parentheses are estimated errors resulting from the measurement of abundance ratios.

The ln[I(Li+-DA2)/I(Li+-DA1)] values for all pairs are consistent internally with a difference not more than

0.2

H2N-(CH2)n-NH2

n = 2-8 (1-7)

ln(I(M+- L2) /I (M+- L1)) ~ E /RTeff~

• Attempts were made to convert relatve orders into relative alkali metal ion affinities by measuring the Teff of the dissociating cluster ions.

• We seek to explain the observed contrasting ordering for H+ and Li+ ion affinities of -diamines through quantum chemical calculations.

Metal ion affinity (H298) = EeleEthermalS - BSSE --2

Proton affinity (H298) = EeleEthermal + 5RT/2 --3

It is well known that, for chemically similar compounds, the natural logarithm of intensity (I) ratio values are directly proportional to the binding energy difference (E) (eq 1) between the used diamines with alkali metal ions (M+), where the entropy term is close to zero.

B3LYP/6-311++G** optimised geometries of cyclic H+ and Li+ ion complexes of diamines. Bond lengths in Å and bond angles in degrees.

Theoretically obtained H+ and Li+ ion affinity orders can be given as

1H+ < 2H+ < 7H+ < 6H+ ≤ 4H+ < 5H+ < 3H+ and 1Li+ < 3Li+ ≤ 2Li+ < 4Li+ < 6Li+ < 5Li+ ≈ 7Li+

Present study addresses this topic by assessing the Li+, Na+, and K+ affinities of the -diamines.

[DA1--Li+--DA2]

DA2 + DA1Li+ (7)

DA2Li+ + DA1 (8)

1H+ < 2H+ < 7H+ ≤ 6H+ < 5H+ < 4H+ < 3H+

1Li+ < 3Li+ ≤ 2Li+ < 4Li+ < 6Li+ < 5Li+ ≤ 7Li+

1Na+ < 2Na+ < 3Na+ < 4Na+ < 5Na+ < 6Na+ < 7Na+

2K+ < 1K+ < 3K+ < 4K+ < 6K+ < 5K+ < 7K+

Proton and alkali metal ion affinities of Proton and alkali metal ion affinities of ,,--diols:diols:

Spacer chain length effectsSpacer chain length effects• The alkali metal ion affinity orders of ,-diamines were compared with their

proton affinity order and found that the affinity orders depend on the size of the central ion used as well as the spacer chain length of ,-diamine.

• It is always ideal to extend such kind of gas phase ion studies to other bifunctional group molecules for better understanding of their multiple interactions with proton/metal ions.

• The Li+, Na+ and K+ ion affinity order of a series of ,-diols (HO-(CH2)n-OH, n= 2-10, 8-16) can be measured by the Kinetic method

HO (CH2)n OH M+

OH

(CH2)n

HO

M+

Measured ln[I(H+-Diol2)/I(H+-Diol1)] values for H+-bound heterodimers of diols (8–16). The data

presented under the heading ln[I(H+-Diol2)/I(H+-14)] are average cumulative values expressed

relative to octane diol (14).

• The relative affinity order for proton is 8H+<< 9H+<< 14H+ ≈ 13H+< 12H+< 11H+< 10H+< 15H+< 16H+

• where as for alkali metal ions the affinities are in the order of 8M+<< 9M+< 10M+< 11M+< 12M+< 13M+< 14M+< 15M+< 16M+, irrespective of alkali metal ion used.

• The overall proton/alkali metal ion affinity orders of diols is almost similar to that obtained for diamines, except some dissimilarities for the Li+ ion affinity order of diamines.

CHAPTER 3Generation of regiospecific carbanions from aromatic hydroxy acids and dicarboxylic acids and characterized ion-molecule reactions with

carbon dioxide

؟ Why the study of carbanions in the gas phase is needed?

؟ Will the stable carbanions produce in ESI conditions?

• Carbanions execute a broad and substantial role as reactive intermediates in organic reaction chemistry

• In the absence of solvation, gas phase studies can reveal the details of reaction mechanisms and reactivity of ionic and neutral species

• Only three methods are possible to generate the carbanion.1. Proton Abstraction2. Fluoro desilylation 3. Decarboxylation

1. Proton abstraction methodProton abstraction from R–H by use of a strong base B.Limitations:The precursor must be sufficiently acidic deprotonation is Limited to molecules

with proton affinities (PA) less than 404 kcal/mol.

2. Fluorodesilylation method DePuy and co-workers developed fluorodesilylation reactions for the formation

of carbanions and hence it has become popular as the DePuy reaction.

3. Decarboxylation

Danikiewicz et al.: Phenide ions from the carboxylate anions by using high cone voltage.

COO-

HighCone

Ion-MoleculeRxns with CO2

-

Benzoate ion Phenide ion

Here we describe the results concerning selective formation of very unstable regiospecific carbanion from isomeric compounds.

Danikiewicz et al. generated and studied the carbanions under ESI conditions.

The detection of the Carbanions is very easy, because they easily reacts with CO2.

Chou and Kass produced geometrical isomeric vinyl carbanions and studied differences in the reactivity of these isomers by ion-molecule reactions.

OH

O

O

OH

O-

O

O

OH

O

O

HH

-

O

O-

H H

O

OH

H-

O

OH

-O

O

O

OH

HO

O

1

2

m/z 115 1C, m/z 71

2C, m/z 71m/z 115

I, m/z 71

M [M-H]- [M-H-CO2]--Ve ESI -CO2

Y

HOOC

XHX = COO, Y = CH, 5X = COO, Y = N, 8X = O, Y = CH, 12

5C/8C/12CY XH

-

XH

-OOC

Y

Y

HOOC

X-

II/III/IV

Y XHHOOC

X = COO, Y = N, 9X = O, Y = N, 14

9C/14CY XH

-OOC Y XH

-

III/VY X

-HOOC

COOH

XHYX = COO, Y = CH, 4X = COO, Y = N, 7X = O, Y = CH, 11

COOH

X-

Y

II/III/IV

X = COO, Y = CH, 3X = COO, Y = N, 6X = O, Y = CH, 10X = O, Y = N, 13

Y

COOH

XH

Y

COO-

XH

3C/6C/10C/13C

YH

X

-

X = COO, Y = CH, IIX = COO, Y = N, IIIX = O, Y = CH, IVX = O, Y = N, V

Y

COOH

X-

Y X-

COO-

XHY4C/7C/11C

YH

X

-

m/z 151

m/z 151

m/z 151

m/z 151

m/z 151

m/z 151

M [M-H]- [M-H-CO2]--Ve ESI -CO2

15

CH2COO-

OH

O-

CH2COOH

16C, m/z 107

17C, m/z 107

CH2COOH

OH

16

17

CH2COOH

OH

OH

CH2COOH

CH2COOH

O-

CH3

O-

CH2-

OH

O-

CH3

15C, m/z 107

CH2COO-

OH

CH2-

OH

CH2COOH

O-

CH3

O-

OH

CH2COO-

OH

CH2-

VIa, m/z 107

VIb, m/z 107

VIc, m/z 107

At high desolvation temperatures (3000C), instead of 1000C, the relative abundance of [(M–H)–CO2]- ions and the corresponding CO2 adduct in ion-molecule reaction experiments increased significantly due to minimization of proton exchange

CompoundSource/

Desolvation Temp (OC)

% increase in yield

1100/100

8.5150/300

3100/100

25.3150/300

4100/100

12.9150/300

11100/100

17.2150/300

12100/100

2150/300

= O= = HC

1C-TS PR-I 2C-TS

1.353

1.224

1.0221.837 1.346

1.230

1.1421.546

1.362

1.221

0.9781.320

1.222

1.0151.334

1.538

1.259

1.254

IC 2C

1.488 1.3481.506 1.322 1.508 1.346

2.1302.334

1.480 1.372

1.412

1.408

1.400

1.394

1.398

1.400

1.507

1.330

1.067

1.407

1.4011.400

1.394

1.3971.509

1.2241.327

1.084

1.646

3C 3C-TS

1.223

1.411

PR -II

1.5541.399

1.395

1.397

1.396

1.399

1.396

1.253

1.435

1.380

1.4311.431

1.560

1.304

1.467

1.4351.410

1.393

1.4231.424

1.391

1.4091.456

1.223

1.383

0.967

5C-TS 5C

3.726

1.217

1.4871.414

1.410

1.4161.404

1.390

1.403

1.212 1.2911.521

1.465

1.438

1.4101.398

1.414

1.515

2.400

4C-TS4C

1.372

1.369

1.401

1.404

1.4101.394

1.401

1.396

1.399

1.385

1.4211.392

1.409

1.3981.342

1.282

1.479

10-C 10C-TS

0.972

1.417

PR -IV

1.2691.447

1.388

1.4041.404

1.388

1.447

1.392

1.408

1.4131.415

1.407

1.390

1.403

1.462

1.381

1.4571.457

1.381

1.462

1.3221.367

1.650

12C-TS 12C

0.963

1.4021.399

1.411

1.4201.399

1.400

1.391

1.401

2.714

1.439

1.416 1.408

1.405

1.3961.321

11C-TS11C

0.963

1.452

Optimized geometries at B3LYP/6-311++G** level

Quantum chemical calculations on some of the generated isomeric carbanions and their isomerised products due to proton transfer

Quantum chemical calculations

Structure 1C 2C 3C 4C 5C 10C 11C 12C

E# 0.8 18.1 0.01 37.5 50.1 18.3 58.3 105.4

ER-

33.3-

42.9-

34.4-53.4

-51.3

-40.0 -51.2 -53.8

Generation of regiospecific carbanions from Sulfobenzoic acidsGeneration of regiospecific carbanions from Sulfobenzoic acids

• Here we have selected isomeric sulfobenzoic acids and disulfonic acids

COOH

SO3H

COOH COOH

SO3H

SO3H18 19 20

SO3H

SO3H

SO3H

SO3H

21 22

-Ve ESI

- CO2

18

19 20

SO3-

COOH

SO3H

COO-

SO3HCOOH

SO3-

COOH

SO3-

SO3H

COOH

SO3-

COOH SO3H

COOH

SO3H

COO-

SO3-

COOH

SO3H

COO-

SO3H

COO-

Ion-Molecule Reactions with CO2

SO3H

-

SO3H

-

SO3HCOO-

-Ve ESI

-Ve ESI

-Ve ESI-Ve ESI

-Ve ESI - CO2

- CO2 - CO2

- CO2- CO2

m/z 201

m/z 201

m/z 201

m/z 201

m/z 201

m/z 201

m/z 201 m/z 201

I, m/z 157

A, m/z 157 B, m/z 157

3

SO3H

COOHSO3H

COO-

SO3-

COOH

-Ve ESI

-Ve ESI

COO-

-SO3

-SO3

m/z 121

m/z 201

m/z 201m/z 237m/z 237 I, m/z 15721 22

-Ve ESI -Ve ESI

SO3HSO3H

SO3-

SO3HSO3

- SO3-

SO3H

SO3H

SO3H- SO3 - SO3

Schemes:

Schemes 1

Schemes 2

Schemes 3

GENERATION OF DISTONIC DEHYDROPHENOXIDE RADICALANIONS UNDER ELECTROSPRAY AND ATMOSPHERIC PRESSURE

CHEMICAL IONIZATION CONDITIONS

General methods for the Preparation of radical anions

1. Electron attachment (dissociative)

2. Electron transfer

3. Ion-molecule reactions

N2O N2 + O-.e

DISTONIC RADICAL ANIONS Definition:Distonic Ions: which possess distinct, spatially separated charged and radical sites.

OH

R

O-

R

O-

.OH -

-R.

R = H/Me/Et/ i-Pr m/z 92Distonic dehdyro phenoxide

radical anions

Bowie et al.

Squires and co-workers presented several applications of the above method to generate isomeric distonic radical anions

Si(CH3)3

-(CH3)3SiFSi(CH3)3

Si(CH3)3

- -

.

F -

- (CH3)3SiF

-F -, -F .

F2

m/z 76o-, m- and p-

O--CO2

NO2

CO2-

-NO.

SORI-CID SORI-CIDNO2

- .

EI/ESINO2

CO2H

m/z 92 o-, m- and p-Nitrobenzoic Acid

COOH

COOH

COO-

COO- COO-

.

-

.

F -

HF

SORICO2

SORI

o-, m- and p-benzenedicarboxylic acid

-CO2

Kass et al. recently reported another new method for the generation of distonic radical anions from aromatic mono and dicarboxylic acids

Characterization of radical anions: Include isotopic labeling, specific ion-molecule reactions, CID, and collision induced

charge reversal processes

.

CO2

.

CO2-

NO2

CO2-

NO2

-

• In Chapter 3, we have shown that isomeric carbanions do survive in the ESI process and selectively react with CO2 when ion-molecule reactions are performed on these carbanions in the collision cell.

• This encouraged us to extend the same method to study generation of isomeric dehydrophenoxide radical anions from suitably substituted nitrobenzoic acids and phenols, and studying their ion-molecule reactions with CO2 in the collision cell.

Nitrobenzoic acids

COOH

NO2

COOH

NO2

COOH

NO2

1 2 3

10 ev

20 ev

30 ev

40 ev

Negative ion electrospray ionization spectra of 3 at different cone voltage values

CID mass spectra of (a) [3-H]‑ (m/z 166) at 20 eV collision energy, (b) [3-H-CO2]‑ (m/z 122) at 20 eV collision energy.

COOH

NO2

COOH

NO2

COOH

NO2

COO-

NO2

COO-

NO2

COO-

NO2

NO2

NO2

NO2

-

-

-

O-

O-

O-

.

.

.

1

2

3

I, m/z 92

II, m/z 92

III, m/z 92m/z 166 m/z 122

m/z 122

m/z 122

m/z 166

m/z 166

-ve ESI

-ve ESI

-ve ESI

-NO.

-NO.-CO2

-NO.-CO2

-CO2

Mechanism

OH

CH3

OH

CH3

OH

CH34 5 6

OH

CHO

OH

CHO

OH

CHO

OH

NO2

OH

NO2

OH

NO27 8 9

10 11 12

OH

R

OH

OH

R

R

-Ve ESI

-Ve ESI

-Ve ESI

O-

R

O-

O-

R

R

-R.

-R.

-R.

O-

.

O-

O-

.

R = -CH3 (4) ; -NO2(7)

R = -CH3 (5) ; -NO2 (8); -CHO(11)

R = -CH3 (6) ; -NO2 (9); -CHO (12)

.

• The compound 10 does not yield the expected ion at m/z 92, instead it shows the ion at m/z 93 corresponding to the loss of CO from [M-H]- ion due to ortho-effect

ESI-high resolution mass spectrum of compound 12.

The Ion-molecule reactions mass spectra of m/z 92, [(12-CHO)-NO]-. with CO2 in the collision cell

Generation of Distonic dehydrophenoxide radical anions from Generation of Distonic dehydrophenoxide radical anions from substituted phenols undersubstituted phenols under atmospheric pressure chemical ionization atmospheric pressure chemical ionization conditions.conditions.

NO2

CHO

NO2

CHO

NO2

CHO

NO2

COCH3

NO2

COCH3

NO2

COCH3

13 14 15

16 17 18

• Though ESI technique is not amenable to study the isomeric nitrobenzaldehydes and nitroacetophenones, they can be analyzed under negative ion APCI conditions

• Loss of NO˙ from the molecular ions of nitroaromatic compounds generated under EI conditions was reported using a tandem sector mass spectrometer.

• In this part, two groups of isomeric substituted nitrobenzenes (13-18), i.e. ortho-, meta- and para- nitrobenzaldehydes (13-15) and ortho-, meta- and para- nitroacetophenones (16-18) were selected to study their source fragmentation under APCI conditions.

• Under APCI conditions the studied compounds form M-. ion, and upon source fragmentation/CID they result in [M-NO]- ion.

• Further fragmentation of the [M-NO]- of ortho-isomers specifically show loss of a neutral (CO or COCH2) to yield the fragment ion at m/z 93.

• The [M-NO]- of meta- and para- isomers further show a radical loss (.CHO or .COCH3) to generate dehydrophenoxide radical anion (m/z 92).

NO2

NO2

CHO

CHO

-Ve APCI

-Ve APCI

O-

O-

CHO

CHO

-CHO.

O-

.

O-

.

NO2

NO2

CHO

CHO

-.

-.

-NO.

-NO.

14

15

m/z 92

m/z 92

-CHO.

Generation of Distonic dehydrophenoxide radical anions from Generation of Distonic dehydrophenoxide radical anions from substituted phenols undersubstituted phenols under atmospheric pressure chemical ionization atmospheric pressure chemical ionization conditions.conditions.