spin−spin coupling constants for iminoboranes rbnh, hbnr, and rbnr and comparisons with...

Spin -Spin Coupling Constants for Iminoboranes RBNH,HBNR, and RBNR and Comparisons with Corresponding

Isoelectronic Acetylenes RCCH and RCCR, for R ) H,CH3, NH2, OH, and F

Janet E. Del Bene,*,† JoseElguero,‡ Ibon Alkorta,‡ Manuel Yanez,§ and Otilia Mo§

Department of Chemistry, Youngstown State UniVersity, Youngstown, Ohio 44555,Instituto de Quı´mica Medica, Consejo Superior de InVestigaciones Cientı´ficas,

Juan de la CierVa, 3, E-28006 Madrid, Spain, and Departamento de Quı´mica, C-9,UniVersidad Auto´noma de Madrid, Cantoblanco, E-28049 Madrid, Spain

Received December 5, 2006

Abstract: Ab initio equation-of-motion coupled-cluster singles and doubles method calculations

have been performed on iminoboranes RBNH, HBNR, and RBNR, for R ) H, CH3, NH2, OH,

and F, to evaluate substituent effects on one- and two-bond 15N-11B, 11B-1H, and 15N-1H

spin-spin coupling constants. For comparison purposes, 13C-13C coupling constants were

evaluated for corresponding isoelectronic molecules RCCH and RCCR. The absolute values of1J(N-B) and 1J(C-C) increase in the series HBNR and RCCH as the σ-electron-withdrawing

ability of R increases. In contrast, NH2 substitution at B leads to a decrease in the absolute

value of 1J(N-B), but OH and F substitution increase 1J(N-B). Disubstitution has dramatically

different effects on coupling constants for RCCR and RBNR. In the former, 1J(C-C) more than

doubles relative to the corresponding RCCH and HCCH; in the latter, disubstitution of OH and

F decreases 1J(N-B) relative to the corresponding monosubstituted derivatives, while NH2

substitution increases 1J(N-B). Changes in one- and two-bond B-H and N-H coupling constants

upon substitution are similar to changes observed for 1J(N-B) in the corresponding monosub-

stituted derivatives RBNH and HBNR.

IntroductionThe examination of substituent effects on molecular proper-ties has proven to be a very fruitful approach in a wide rangeof studies of organic molecules.1-5 In a previous study,6 weused this approach to examine the relative stabilities ofcorresponding isomers in two series of iminoboranes RBNHand HBNR, where R is H, CH3, NH2, OH, and F. Weinvestigated ground-state bonding and selected IR spectro-scopic properties of these molecules, as well as the bondingcharacteristics and IR properties of their disubstitutedcounterparts, RBNR. The present paper reports the results

of an extension of our investigation of iminoboranes toinclude substituent effects on15N-11B spin-spin couplingconstants in molecules RBNH, HBNR, and RBNR, as wellas one- and two-bond15N-1H and 11B-1H spin-spincoupling constants in the monosubstituted derivatives. Wealso present13C-13C coupling constants for RCCH andRCCR, molecules that are isoelectronic with correspondingRBNH and HBNR, and RBNR, respectively. In this paper,we compare and contrast substituent effects of one-bondB-N coupling constants in mono- and disubstituted imi-noborazines and compare them to substituent effects on C-Ccoupling constants in the corresponding mono- and disub-stituted acetylenes.

The present study is also an extension of our previousinvestigation of B-N coupling constants in borazine and itsfluoro- and lithio-substituted derivatives.7 In that study, we

* Corresponding author e-mail: [email protected].† Youngstown State University.‡ Consejo Superior de Investigaciones Cientı´ficas.§ Universidad Auto´noma de Madrid.

549J. Chem. Theory Comput.2007,3, 549-556

10.1021/ct600351p CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 02/09/2007

examined the sensitivity of1J(N-B) to the site of substitutionand the number of substituents and also observed theopposing effects ofσ-electron-donating and -electron-withdrawing substituents, since F substitution at either B orN increases1J(N-B), whereas Li substitution decreases1J(N-B). These substituent effects in borazine are similarto those observed for benzene and are consistent withavailable experimental data.7 Recently, we have also reportedone-bond B-H and B-Li coupling constants for the five-membered boron-containing rings diazaborole and Li-diaz-aborole, as well as B-N coupling constants for thesemolecules and the corresponding diazaborole anion,8 specieswhich served as models for newly synthesized five-membered nucleophilic boron-containing rings.9 NMR cou-pling constants and chemical shifts were used experimentallyto identify these species. It is anticipated that the newlydeveloped synthetic techniques which produced nucleophilicboron moieties will lead to the synthesis of a new class ofmolecules containing B-N bonds and that NMR spin-spincoupling constants will be an important tool for the identi-fication and characterization of these molecules. Hence, asystematic study of substituent effects on B-N couplingconstants is both warranted and timely.

MethodsThe geometries of the molecules RBNH, HBNR, and RBNRwere optimized at second-order Møller-Plesset theory10-13

with the 6-311++G(d,p) basis set14-16 and have beenreported previously.6 Coupling constants were computed forthese molecules using the ab initio equation-of-motioncoupled-cluster singles and doubles method (EOM-CCSD)in the configuration-interaction- (CI-)like approximation,17-20

with all electrons correlated. The Ahlrichs qzp basis set21

was used on C, N, O, and F atoms, and the qz2p basis setwas used for H atoms bonded to either B or N. The Dunningcc-pVDZ22,23basis set was placed on all other H atoms. Sincean Ahlrichs qzp basis is not available for B, a new “hybrid”basis had been constructed for this atom, using the boroncc-pV5Z basis for 1s orbitals, cc-pVQZ for the valence sand p orbitals, and one set of d orbitals. This basis set hasthe same number of contracted functions (6s, 4p, and 1d) asthe Ahlrichs qzp basis used for C, N, O, and F and was usedpreviously in studies of B-N, B-H, and B-Li couplingconstants.7,8

In the nonrelativistic approximation, the nuclear spin-spin coupling constant is composed of four terms: theparamagnetic spin-orbit (PSO), diamagnetic spin-orbit(DSO), Fermi-contact (FC), and spin-dipole (SD).24 All termshave been computed for all molecules. The EOM-CCSDcalculations were carried out using the ACES II25 programon the Itanium Cluster at the Ohio Supercomputer Center.

Results and DiscussionSubstitution Effects on 1J(C-C): RCCH and RCCR.Before analyzing the B-N coupling constants as a functionof the substituent, it is advantageous to first examine C-Ccoupling constants for molecules RCCH, which are isoelec-tronic with corresponding molecules RBNH and HBNR, andfor RCCR, which are isoelectronic with the corresponding

RBNR. The individual contributions of the PSO, DSO, FC,and SD terms to1J(C-C) can be found in Table S1 of theSupporting Information. The FC terms, total1J(C-C), andC-C distances are reported in Table 1. The Fermi-contactterm is the dominant term, although contributions from thePSO terms (ranging from 6 to 15 Hz) and from the SD terms(8 to 10 Hz) cannot be neglected. Moreover, although thedata of Table 1 indicate that1J(C-C) has its maximumvalues in the RCCH and RCCR series when R is F and theC-C distances are shortest,1J(C-C) does not correlate withthis distance. For example, although the C-C distance inH2NCCH is intermediate between that in HCCH and that inH3CCCH, differing from both by only 0.001 Å,1J(C-C)for H2NCCH is 34 Hz greater than1J(C-C) for HCCH andH3CCCH.

The only experimental coupling constants available for themolecules investigated in this study are for acetylene (170.6Hz)26 and methylacetylene (175 Hz).27 From Table 1, it canbe seen that the computed EOM-CCSD values of 192.3 and192.7 Hz, respectively, overestimate the experimental valuesby about 10%. This may be due in part to the sensitivity of1J(C-C) to the length of the C-C bond, as demonstratedby Wigglesworth et. al,28 and also to the neglect of zero-point and thermal vibrational effects.29-32 However, ouremphasis in this study is on changes in coupling constantsdue to the substitution ofσ-electron-withdrawing groups, andthe results reported below for substituted acetylenes are givenonly for comparison with substituent effects in iminoboranes.Nevertheless, as will be evident below, our results forsubstituent effects on1J(C-C) are in agreement with resultsreported by Kamienska-Trela and co-workers, who havecarried out extensive studies of substituent effects on C-Ccoupling constants.27,33

What happens to1J(C-C) upon the substitution of H bythe ground-stateσ-electron-withdrawing substituents CH3,NH2, OH, and F? As evident from Table 1,1J(C-C) valuesfor HCCH, H3CCCH, and H3CCCCH3 are similar. However,as theσ-electron-withdrawing ability of the substituents

Table 1. C-C Fermi-Contact Terms and CouplingConstants [1J(C-C), Hz], C-C Distances (Å), andSymmetries for Molecules RCCH and RCCR

Coupling Constants

RCCH RCCR

molecule FC 1J(C-C) FC 1J(C-C)

R ) H 178.1 192.3 178.1 192.3CH3 178.5 192.7 180.9 195.2NH2 210.8 226.3 253.7 270.1OH 232.8 250.2 313.6 334.0F 258.8 278.7 394.1 418.8

C-C Distances and Molecular Symmetry

molecule RCCH RCCR

R ) H 1.216 (C∞v) 1.216 (D∞h)CH3 1.218 (C3v) 1.220 (D3h)NH2 1.217 (Cs) 1.217 (C2)OH 1.214 (Cs) 1.208 (C2)F 1.208 (C∞v) 1.197 (D∞h)

550 J. Chem. Theory Comput., Vol. 3, No. 2, 2007 Del Bene et al.

increases in the order CH3 < NH2 < OH < F, 1J(C-C)increases in the same order in both the mono- and disubsti-tuted derivatives. Figure 1 graphically illustrates the linearcorrelation between1J(C-C) for RCCR and1J(C-C) forRCCH. As theσ-electron-withdrawing ability of the sub-stituent increases,1J(C-C) increases, primarily because ofan increase in the FC term. A second very interestingobservation is the slope of 2.6 for the trend line shown inFigure 1, which indicates that disubstitution is nonadditivein a positive sense, insofar as it more than doubles1J(C-C)in RCCR relative to the corresponding RCCH and HCCH.These observations indicate that1J(C-C) increases system-atically as theσ-electron-withdrawing ability of the R groupincreases.

How can these results be explained? Because1J(C-C) isdominated by the FC term, some insights can be obtainedby considering the nature of this term and the nature of theexcited triplet-state wavefunctions which couple to theground state through the FC operator. First, it should berecalled that the FC operator does not contain a specificdistance-dependent term. Second, if the function of the Rgroups is to withdrawσ-electron density from the C-C bondin the ground state, then the increase in1J(C-C) as theσ-electron-withdrawing ability of the substituent increasessuggests that the excited states which dominate must haveincreasedσ-electron densities on the two carbon atoms.Alternatively, in going from NH2 to OH to F, the number ofnonbonding pairs of electrons on the substituent increases.These lone pairs can interact with the C-C π bonds and,from a sum-over-states perspective, influence both the signand magnitude of the contributions to1J(C-C) from thevarious excited states that couple to the ground state. Sincethe FC term dominates and the sign of1J(C-C) is positive,

triplet excited states with an odd number of nodes intersectingthe C-C bond assume increased importance.34 The effectof σ-electron-withdrawing substituents on1J(C-C) in thesubstituted acetylenes is consistent with the effect of Fsubstitution on one-bond C-C coupling constants in fluo-robenzene, which also increase when coupling involves thefluoro-substituted C atom.7

1J(N-B) and B-N Distances.Molecular symmetries,B-N distances, Fermi-contact terms, and one-bond B-Ncoupling constants [1J(N-B)] for the entire set of moleculesRBNH, HBNR, and RBNR are reported in Table 2. Theindividual contributions of PSO, DSO, FC, and SD termscan be found in Table S2 of the Supporting Information. Asobserved previously for borazine,7 the FC term dominates1J(N-B), accounting for more than 90% of the total couplingconstant in all cases. The data of Table 2 also indicate thatthere is little correlation between one-bond B-N couplingconstants and B-N distances. For example, among thedisubstituted iminoboranes, the B-N distances are shortestin HBNH and H3CBNCH3, but these two molecules havethe smallest B-N coupling constants. Thus, it can beconcluded that, in general, changes in B-N couplingconstants in substituted iminoboranes do not depend simplyon changes in B-N distances. This observation is consistentwith previous theoretical and experimental data on couplingconstants and distances for molecules35,36 and is really notsurprising in view of the absence of a distance-dependentterm in the Fermi-contact operator.

Substituent Effects on1J(N-B). Having examined1J(C-C) as a function of the substituent for RCCH and RCCR, itis appropriate to now return to1J(N-B) for RBNH, HBNR,and RBNR. The B-N coupling constants for these moleculesare reported in Table 2. As observed for CH3 substitution

Figure 1. 1J(C-C) for RCCR (blue [, left y axis) and 1J(B-N) for HBNR (red 9, right y axis) vs 1J(C-C) for RCCH.

Spin-Spin Coupling Constants for Iminoboranes J. Chem. Theory Comput., Vol. 3, No. 2, 2007551

on 1J(C-C), substitution of one CH3 group at either B orN, or two CH3 groups, one at B and one at N, has little effecton 1J(N-B). Moreover, in the series HBNR, substitution ofthe strongerσ-electron-withdrawing substituents NH2, OH,and F increases the absolute value of1J(N-B) in the orderCH3 < NH2 < OH < F, consistent with the RCCH series.The correlation between substituent effects on1J(N-B) inthe series HBNR and1J(C-C) for RCCH is also illustratedin Figure 1. [It should be noted that, since the magnetogyricratio of 15N is negative, while that of11B is positive, theone-bond B-N coupling constant1J(N-B) is negative, butthe reduced one-bond coupling constant1K(N-B) is positive,as is1K(C-C).]

In contrast, a correlation between1J(C-C) and1J(N-B)is not observed for substitution at B in the series RBNH. Inparticular, NH2 substitution at B leads to a decrease of1J(N-B) relative to HBNH, while OH and F substitution increase1J(N-B). [Changes in1J(N-B) will be discussed in termsof absolute values.] Thus,1J(N-B) is extremely sensitiveto the site of substitution when the substituent is NH2 but isrelatively insensitive to which site is substituted when R isOH or F. This is illustrated graphically in Figure 2, a plot of1J(N-B) for HBNR versus 1J(N-B) for RBNH. Thereference line shown passes through the origin and has aslope of 1. Relative to the other molecules in the two seriesof monosubstituted derivatives,1J(N-B) for H2NBNHappears to be too small; that is, NH2 bonded to B acts morelike H and CH3 rather than the strongerσ-electron-withdraw-ing substituents OH and F. Since1J(N-B) for HBNR islinearly correlated with1J(C-C), it is apparent that no suchcorrelation exists for coupling constants between RBNH andRCCH.

Why does the substitution of NH2 at B fail to produce theexpected increase in1J(N-B)? Some insight into the answerto this question can be gained by considering the nature ofthe BN bond in H2NBNH. At the outset, it must be statedthat coupling constants are second-order properties thatdepend on electron densities in both the ground state andthe excited states to which they couple, and any explanation

of changes in coupling constants based on ground-stateproperties must be incomplete. However, it is not unreason-able to suggest that, in a series of related molecules,differences in ground-state bonding characteristics may bemanifested by differences in coupling constants. In ourprevious study of the ground-state bonding characteristicsof substituted iminoboranes,6 we concluded that, whilesubstitution at N only slightly perturbs the BN triple bond,NH2 substitution at B has a rather dramatic effect on thisbond. In H2N-BNH, there is a strong contribution to theBN bond from a canonical form of the type H2NdBdN-H, which indicates that the BN bond loses triple bondcharacter. This is associated with a significant increase inthe B-N distance relative to all other monosubstitutedderivatives and a value of1J(N-B) that is much less thanexpected, as clearly illustrated in Figure 2.

The effect of disubstitution of NH2, OH, and F on1J(N-B) is dramatically different from the effect of monosubsti-tution. In particular, disubstitution of NH2 increases1J(N-B) relative to both H2NBNH and HBNNH2, but disubstitutionof OH or F decreases1J(N-B) relative to the correspondingmonosubstituted derivatives RBNH and HBNR. Specifically,1J(N-B) decreases from-112 Hz in HOBNH and HBNOHto -96 Hz in HOBNOH, and from-123 and-128 Hz inFBNH and HBNF, respectively, to-107 Hz in FBNF, while1J(N-B) increases from-86 and-103 Hz in H2NBNH andHBNNH2, respectively, to-116 Hz in H2NBNNH2. Thesedifferences are also shown graphically in Figure 2, where1J(N-B) for RBNR is plotted against1J(N-B) for RBNH.Once again, the reference line passes through the origin andhas a slope of 1. The corresponding plot for RBNR versusHBNR has a similar appearance. It is apparent that disub-stitution of NH2 leads to a dramatic increase in1J(N-B)relative to HBNNH2 and H2NBNH, whereas disubstitutionof OH or F actually decreases1J(N-B) relative to thecorresponding monosubstituted derivatives.

Some insight into at least one factor which influences thisrather strange behavior can be obtained by comparing1J(N-B) for RBNR and1J(C-C) for RCCR. Figure 3 indicates

Table 2. B-N Fermi-Contact Terms and Coupling Constants [1J(N-B), Hz]; B-N Distances (Å); and Symmetries forMolecules RBNH, HBNR, and RBNR

Coupling Constants

RBNH HBNR RBNR

molecule FC 1J(N-B) FC 1J(N-B) FC 1J(N-B)

R ) H -84.9 -89.4 -84.9 -89.4 -84.9 -89.4CH3 -84.1 -88.2 -84.9 -89.9 -84.3 -89.0NH2 -83.4 -86.4 -96.9 -103.4 -110.1 -115.6OH -108.5 -112.6 -104.5 -112.4 -91.0 -96.1F -118.3 -123.1 -118.1 -127.7 -99.5 -106.6

B-N Distances and Molecular Symmetry

molecule RBNH HBNR RBNR

R ) H 1.247 (C∞v) 1.247 (C∞v) 1.247 (C∞v)CH3 1.250 (C3v) 1.250 (C3v) 1.252 (C3v)NH2 1.264 (Cs) 1.248 (Cs) 1.254 (C1)OH 1.250 (Cs) 1.246 (Cs) 1.268 (C1)F 1.245 (C∞v) 1.241 (C∞v) 1.260 (Cs)


that, while disubstitution of CH3 has very little effect on thesetwo one-bond coupling constants, there is no correlationbetween1J(N-B) for RBNR and1J(C-C) for RCCR when

R is NH2, OH, and F. From Figure 1, it is apparent thatdisubstitution of OH and F in the acetylenes significantlyincreases1J(C-C) for HOCCOH and FCCF relative to

Figure 2. 1J(N-B) for HBNR (green [) and 1J(N-B) for RBNR (red 9) vs 1J(N-B) for RBNH. The reference line has a slopeof 1 and passes through the origin. The points corresponding to NH2, OH, and F as substituents are marked by arrows. Thepoints for H and CH3 are at the bottom left of the graph.

Figure 3. 1J(N-B) for RBNR vs 1J(C-C) for RCCR.


HOCCH and FCCH, respectively. In contrast, disubstitutionof OH and F in the iminoboranes actually decreases1J(N-B) relative to either of the corresponding monosubstitutedderivatives. The opposite effects of these two substituentson 1J(N-B) and 1J(C-C) are consistent with our earlier

observations that disubstitution of OH and F have oppositeeffects on C-C and B-N bonds.6 Disubstitution of thesetwo groups appears to strengthen and shorten the C-C bondrelative to the corresponding monosubstituted derivatives butlengthen the B-N bond relative to either of the correspond-ing monosubstituted derivatives, as evident from Tables 1and 2, respectively. What remains unclear is why substitutionof two NH2 groups in iminoborane should increase1J(N-B) relative to either monosubstituted derivative, therebyhaving a similar effect to mono- and disubstitution of NH2

in acetylene.

Substitution Effects on B-H and N-H CouplingConstants.Table 3 presents one- and two-bond B-H andN-H Fermi-contact terms and coupling constants andcorresponding distances for the monosubstituted derivativesRBNH and HBNR. The contributions of all terms to thesecoupling constants are reported in Table S2 of the SupportingInformation. Once again, the FC term dominates, contributingmore than 90% of the coupling constant except for two-bondN-H coupling in HBNR molecules, in which case2J(N-H) is relatively small, ranging from-15 to -19 Hz. One-bond N-H distances in RBNH and B-H distances in HBNRshow little sensitivity to substitution, varying by only 0.004and 0.003 Å, respectively, in these two series. Despite thesmall variation in N-H and B-H distances,1J(N-H) variesfrom -104 to-122 Hz in the series RBNH, while1J(B-H) varies from 230 to 265 Hz in the series HBNR.

Patterns similar to those observed for B-N couplingconstants in the two series RBNH and HBNR are alsoobserved for one- and two-bond B-H and N-H couplingconstants. Both B-H and N-H coupling constants arerelatively insensitive to the replacement of H by CH3. With

Table 3. One- and Two-Bond B-H and N-H CouplingConstants and Fermi-Contact Terms (Hz) and B-H andN-H Distances (Å) for Molecules RBNH and HBNR

Coupling Constants

RBNH FC 1J(N-H) FC 2J(B-H)

R ) H 110.3 -111.9 54.8 57.6CH3 -110.3 -112.1 54.5 57.2NH2 -102.1 -104.4 46.0 47.6OH -116.9 -119.1 66.0 68.5F -119.8 -121.8 70.9 73.6

HBNR FC 1J(B-H) FC 2J(N-H)

R ) H 230.7 231.0 -13.9 -15.5CH3 229.2 229.5 -13.2 -15.0NH2 239.4 239.6 -14.4 -16.3OH 249.8 249.9 -15.2 -17.2F 265.0 265.1 -16.9 -18.9

Distances

RBNH HBNR

molecule N-H B-H B-H N-H

R ) H 0.996 2.243 1.168 2.415CH3 0.995 2.245 1.169 2.418NH2 0.997 2.195 1.168 2.415OH 0.993 2.244 1.167 2.412F 0.993 2.239 1.166 2.407

Figure 4. 2J(B-H) vs 1J(N-H) for molecules RBNH.


respect to the substituents, changes in1J(N-H) and2J(B-H) in the series RBNH, and1J(B-H) and 2J(N-H) in theseries HBNR, are similar to changes in1J(N-B) in thecorresponding series. Figure 4 shows a linear correlationbetween2J(B-H) and 1J(N-H) for iminoboranes RBNH.Just as NH2 substitution at B decreases1J(N-B) relative toits value for HBNH, NH2 substitution also decreases1J(N-H) and2J(B-H) relative to HBNH. OH and F substitutionincreases1J(N-H), 2J(B-H), and1J(N-B) relative to thecorresponding coupling constants for the parent moleculeHBNH. B-H and N-H coupling constants in moleculesHBNR are also linearly correlated.

All of the reduced one-bond B-N, B-H, and N-Hcoupling constants are positive in RBNH, HBNR, andRBNR, in agreement with the Dirac vector model,37 whichstates that reduced one-bond coupling constants are positive,two-bond negative, three-bond positive, and so on. However,2J(N-H) values are negative for HBNR, and2J(B-H) valuesare positive for RBNH. Thus, the reduced two-bond couplingconstants2K(N-H) and 2K(B-H) in these series of mol-ecules are both positive and, as such, are exceptions to thisgeneralization.

ConclusionsIn this study, ab initio EOM-CCSD calculations were carriedout on iminoboranes RBNH, HBNR, and RBNR, for R)H, CH3, NH2, OH, and F, to evaluate substituent effects onone- and two-bond15N-11B, 11B-1H, and15N-1H couplingconstants. For comparison purposes, calculations were alsoperformed on corresponding isoelectronic acetylene deriva-tives RCCH and RCCR. The following statements aresupported by the results of these calculations:

1. Mono- or disubstitution of CH3 in iminoborane oracetylene has little effect on B-N or C-C couplingconstants.

2. As theσ-electron-withdrawing ability of the substituentincreases in the order CH3 < NH2 < OH < F, 1J(C-C)increases in both series RCCH and RCCR. The increase in1J(C-C) upon disubstitution is more than double the increaseupon monosubstitution.

3. For the monosubstituted iminoborane derivatives HBNR,1J(N-B) also increases as theσ-electron-withdrawing abilityof the substituent increases in the order CH3 < NH2 < OH< F, as observed for1J(C-C) in the corresponding isoelec-tronic series RCCH.

4. 1J(N-B) does not vary systematically with theσ-electron-withdrawing ability of the substituent in the series RBNH.NH2 substitution decreases1J(N-B) relative to HBNH, whilesubstitution of OH or F leads to similar increases for1J(N-B) in RBNH and HBNR. The sensitivity of1J(N-B) to thesite of NH2 substitution may be related to the significantloss of BN triple-bond character in H2NBNH.

5. The effect of disubstitution in the series RBNR isdramatically different from the effect of monosubstitution.Disubstitution of NH2 increases1J(N-B) relative to eithermonosubstituted derivative, while disubstitution of OH or Fdecreases1J(N-B) relative to the corresponding monosub-stituted derivatives. Disubstitution of OH or F has theopposite effect on coupling constants in RBNR and RCCR.

6. Changes in B-N coupling constants in iminoboranesdo not correlate with changes in B-N distances.

7. Changes in1J(N-H) and 2J(B-H) for moleculesRBNH, and2J(N-H) and1J(B-H) for HBNR, are similarto changes in1J(N-B) for RBNH and HBNR, respectively.Changes in N-H and B-H coupling constants in each seriesare linearly correlated.

Acknowledgment. This work was supported by theUnited States National Science Foundation through GrantCHE-9873815; the DGI Project Nos. BQU-2003-00894,BQU2003-06553, and BQU-2003-01251; and the ProjectMADRISOLAR, ref S-0505/PPQ/0225 of the ComunidadAutonoma de Madrid. The continuing support of the OhioSupercomputer Center is also gratefully acknowledged.

Supporting Information Available: Tables reportingthe PSO, DSO, FC, and SD components of one- and two-bond coupling constants for acetylenes and iminoboranes andthe complete author list for ref 25, are included as SupportingInformation. This material is available free of charge via theInternet at http://pubs.acs.org.

References

(1) Hammett, L. P.J. Am. Chem. Soc.1937, 59, 96.

(2) Taft, R. L.; Lewis, I. C.J. Am. Chem. Soc.1959, 81, 5343.

(3) Exner, O.Collect. Czech. Chem. Commun.1966, 31, 65.

(4) Peyerimhoff, S. D.Interactions in Molecules: Electronic andSteric Effects; Wiley- VCH: Weinheim, Germany, 2003.

(5) Selassie, C. D. The History of Quantitative Structure ActivityRelationships. InBurger’s Medicinal Chemistry and DrugDiscoVery, 6th ed.; Abraham, D. J., Ed.; John Wiley andSons Publishers: New York, 2003; Vol. 1, pp 1-48.

(6) Mo, O.; Yanez, M.; Martin-Penda´s, A.; Del Bene, J. E.;Alkorta, I.; Elguero, J.Phys. Chem. Chem. Phys.(submitted).

(7) Del Bene, J. E.; Elguero, J.; Alkorta, I.; Yan˜ez, M.; Mo, O.J. Phys. Chem. A2006, 110, 9959.

(8) Del Bene, J. E.; Alkorta, I.; Elguero, J.; Yan˜ez, M.; Mo, O.J. Phys. Chem. A2007, 111, 419.

(9) Segawa, Y.; Yamashita, M.; Nozaki, K.Science2006, 134,113.

(10) Pople, J. A.; Binkley J. S.; Seeger, R. Int. J. Quantum Chem.,Quantum Chem. Symp. 1976, 10, 1.

(11) Krishnan, R.; Pople, J. A.Int. J. Quantum Chem.1978, 14,91.

(12) Bartlett, R. J.; Silver. D. M.J. Chem. Phys. 1975, 62, 3258.

(13) Bartlett, R. J.; Purvis, G. D.Int. J. Quantum Chem.1978,14, 561.

(14) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A.J. Chem.Phys.1980, 72, 650.

(15) Spitznagel, G. W.; Clark, T.; Chandrasekhar, J.; Schleyer,P. v. R.J. Comput. Chem.1982, 3, 3633.

(16) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer,P. v. R.J. Comput. Chem.1983, 4, 294.

(17) Perera, S. A.; Sekino, H.; Bartlett, R. J.J. Chem. Phys.1994,101, 2186.

(18) Perera, S. A.; Nooijen, M.; Bartlett, R. J.J. Chem. Phys.1996, 104, 3290.


(19) Perera, S. A.; Bartlett, R. J.J. Am. Chem. Soc.1995, 117,8476.

(20) Perera, S. A.; Bartlett, R. J.J. Am. Chem. Soc.1996, 118,7849.

(21) Scha¨fer, A.; Horn, H.; Ahlrichs, R.J. Chem. Phys.1992,97, 2571.

(22) Dunning, T. H., Jr.J. Chem. Phys.1989, 90, 1007.

(23) Woon, D. E.; Dunning, T. H., Jr.J. Chem. Phys.1995, 103,4572.

(24) Kirpekar, S.; Jensen, H. J. A.; Oddershede, J.Chem. Phys.1994, 188, 171.

(25) Stanton, J. F.ACES II; University of Florida: Gainesville,FL.

(26) Stothers, J. B.Carbon-13 NMR Spectroscopy; AcademicPress: New York, 1972; p 370.

(27) Kamienska-Trela, K.J. Mol. Struct.1982, 32, 143.

(28) Wigglesworth, R. D.; Raynes, W. T.; Kirpekar, S.; Odder-shede, J.; Sauer, S. P. A.J. Chem. Phys.2000, 112, 3735.

(29) Kirpekar, S.; Enevoldsen, T.; Oddershede, J.; Raynes, W.T. Mol. Phys.1997, 91, 897.

(30) Ruden, T. A.; Lutnæs, O. B.; Helgaker, T.J. Chem. Phys.2003, 118, 9572.

(31) Contreras, R. H.; Barone, V.; Facelli, J. C.; Peralta, J. E.Annu. Rep. NMR Spectrosc.2003, 51, 167-260.

(32) Ruden, T. A.; Ruud, K. Ro-Vibrational Corrections to NMRParameters. InCalculation of NMR and EPR Parameters;Kaupp, M., Buhl, M., Malkin, V., Eds.; Wiley-VCH:Weinheim, Germany, 2004; pp 153-173.

(33) Kamienska-Trela, K.; Kania, L.; Schilf, W.; Balova, I.Spectrochim. Acta A1999, 55, 817. Da¸browski, A.;Kamienska-Trela, K; Wo´jcik, J. Spectrochim. Acta A2000,56, 91. Kamien´ska-Trela, K.; Kania, L.; Bernatowicz, P.;Bechcicka, M.; Kaczmarek, L.; Wo´jcik, J.Spectrochim. ActaA 2000, 56, 2079. Kamien´ska-Trela, K.Annu. Rep. NMRSpectrosc.1995, 30, 131.

(34) Del Bene, J. E.; Elguero, J.Chem. Phys. Lett.2003, 382,100.

(35) Del Bene, J. E.; Elguero, J.; Alkorta, I.J. Phys. Chem. A2004, 108, 3662.

(36) Del Bene, J. E.; Elguero, J.Magn. Reson. Chem.2006, 44,784.

(37) Lynden-Bell, R. M.; Harris, R. K.Nuclear MagneticResonance Spectroscopy; Appleton Century Crofts: NewYork, 1969.

CT600351P


spin−spin coupling constants for iminoboranes rbnh, hbnr, and rbnr and comparisons with...

Documents