solvent effects on nmr spectrum of acetylene calculated by ab initio methods

Ž .Chemical Physics 234 1998 111–119

Solvent effects on NMR spectrum of acetylene calculated by abinitio methods

Magdalena Pecul ), Joanna SadlejDepartment of Chemistry, The UniÕersity of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland

Received 27 January 1998

Abstract

The NMR spectrum of acetylene in the gas phase and in the solution was calculated on the SCF and CASSCF level. Thecalculated NMR parameters were studied in relation to the active space dimension and the basis set. Solvent-inducedchanges of acetylene coupling constants were calculated by the reaction field method; for shielding constants thesupermolecular method was also used. The shielding and coupling constants calculated for the isolated molecule showed astrong electron correlation dependence as contrasted with the changes caused by the presence of a dielectric medium.Comparison with the experimental values confirmed an exceptional magnitude of the 1J solvent-induced changes inCC

acetylene. The coupling constants changes calculated by the reaction field method are consistent with the experimentalvalues; the shielding constants changes revealed a large discrepancy. On this basis we conclude that the shielding constantsin solution are affected mainly by the short-range specific interactions while for the coupling constants the long-rangeelectrostatic interactions also contribute significantly. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction

Today ab initio calculations can provide accurateNMR properties of a molecule in the gas state. It iswell known that shielding constants change very

w xconsiderably with the environment 1,2 . The depen-dence of coupling constants on intermolecular inter-actions is, however, often considered negligible inspite of the reported solvent-induced changes ex-

Žw xceeding by far any experimental error 3 and refs.w x.therein, 4 . Investigations on this effect in organic

compounds have been focused mostly on the cou-w xpling constants involving hydrogen 3 . Recent mea-

) Corresponding author.

surements of the C H coupling constants in the gas2 2w xstate and in various organic solvents 5 have shown

1J to be strongly solvent-dependent, too.CC

The aim of this paper is to investigate theoreti-cally the effect of solvent on the NMR parameters ofacetylene. For this purpose two completely differentmethods were used, viz., the reaction field method

Žand the supermolecular method restricted to binary.acetylene–solvent molecule complexes . The former

includes only the description of long-range purelyelectrostatic interactions, whereas the latter describesmainly short-range interactions. This approach madeit possible to establish the terms that play the majorrole in solvent-induced changes of shielding andcoupling constants. Computational limitations re-stricts the use of the supermolecular method for thecalculation of the solvent dependence of coupling

0301-0104r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0301-0104 98 00168-2

( )M. Pecul, J. SadlejrChemical Physics 234 1998 111–119112

Žconstants quite successful in the calculations ofwgas-to-liquid and gas-to-solution chemical shifts 6–

x.8 . In this case only the reaction field theory wasused, except for the smallest system C H –H O.2 2 2

In addition, two most significant methodologicalaspects of ab initio calculations were investigated:the correlation and the basis set effects. The propertheoretical description of the coupling constants re-quires particularly correlation effects to be includedw x 1 139,10 . This is not so essential for the H and Cshielding constants, in which case the SCF method

w x Ž .with GIAO 11,12 gauge invariant atomic orbitalsgives acceptable results for the majority of com-pounds. The dependence of the calculated NMRparameters both in the gas phase and in the dielectricmedium on the correlation level and basis set wasinvestigated. For this purpose, calculations were car-ried out according to the CASSCF scheme withseveral active spaces and two basis sets. All contri-

Ž .butions to the coupling constant: Fermi contact FC ,Ž .paramagnetic spin–orbit PSO , diamagnetic spin–

Ž . Ž .orbit DSO and spin–dipole SD were calculated atthe same level of theory.

2. Method of calculations

In this work acetylene shielding constants werecalculated by the SCF and the CASSCF methods,GIAO orbitals being used in either case. The cou-pling constants of C H were calculated exclusively2 2

by the CASSCF method in various active spaces.The active spaces were chosen according to the MP2natural orbital occupation numbers. They are labelledhere by the number of active orbitals in the differentirreducible representations of the molecule, usingonly the D symmetry. Thus, the notation2hŽ .n n . . . n indicates the number of active orbitals1 2 8

Ž .in the symmetries A B B B B B B A ,g 3u 2u 1g 1u 2g 3g u

respectively. The following abbreviations were used:Ž . Ž .CAS 1 for 3 3 1 1 1 1 0 0 , CAS 2 for 3 3 2 1 2 1 0 0 ,

Ž .CAS 3 for 4 3 2 1 2 1 1 0 and CAS 4 forŽ .4 4 1 1 1 1 0 0 . The inner-shell orbitals of C atomsare inactive, except in the CAS 4 active space. CAS 1active space includes orbitals with MP2 natural or-bital occupation numbers larger than 0.01066, corre-sponding to all orbitals with dominant contributionsfrom carbon and hydrogen valence atomic orbitals.

In CAS 2 two virtual orbitals with MP2 naturalorbital occupation numbers equal 0.01066 are added,CAS 3 was extended by two more orbitals with MP2natural orbital occupation numbers larger than 0.007.CAS 4 corresponds to CAS 1 augmented by inner-shell orbitals of C atoms.

To study the dependence of NMR parameters onthe basis set the HIII and HIV basis sets were used,as developed for the calculations of NMR parameters

w x w xby van Wullen 13 from Huzinaga’s basis sets 14¨w xand frequently employed for these purposes 15 . The

w xHIII basis set consists of 11s 7p 2d r 7s 6p 2d func-w xtions for C and 6s 2p r 4s 2p for H, while the basis

w xHIV is 11s 7p 3d 1f r 8s 7p 3d 1f for C andw x6s 3p 1d r 5s 3p 1d for H. Thus, the total number ofcontracted gaussians is 90 and 140, respectively.

All calculations were carried out in the DALTON

w x Žw xsystem 16 in which the reaction field theory 17.and refs. therein has recently been implemented.

This approach has been already used for the calcula-tion of solvent effects on coupling constants of fluo-roethylene, and found to improve agreement with

w xexperimental data 18 . In the reaction field theorythe solvent is represented as a homogeneous isotropicand linear dielectric medium and the solvatedmolecule is contained within a spherical cavity em-bedded in the medium.

In the reaction field model the cavity radius, theorder of a multipole expansion and the dielectricconstant of the solvent must be defined. In this work6 au was taken for the cavity radius, regardless of thesolvent. This radius corresponds approximately tothe van der Waals radius of the hydrogen atom,

˚varying in nonpolar hydrocarbons from 1.0 to 1.4 Aw x19 plus the distance from the H atom to the centerof symmetry in acetylene. The order of multipoleexpansion was taken equal to 10, thus precluding any

w xadditional errors due to truncation 17 . The dielec-tric constants of solvents are: 2.02 for cyclohexane,2.27 for benzene, 4.7 for chloroform, 5.3 for methy-lene iodide, 11.23 for t-butyl alcohol, 20.7 for ace-tone, 37.5 for acetonitrile and 78.5 for water. Calcu-lations were carried out with the fixed experimental

˚ ˚ w xgeometry R s1.2031 A, R s1.0608 A 20 , un-CC CH

less otherwise noted.Solvent-induced shielding changes were obtained

by the supermolecular method for the following sol-vents: benzene, acetone, acetonitrile chloroform and

( )M. Pecul, J. SadlejrChemical Physics 234 1998 111–119 113

water. The structures of stable binary acetylene–solvent molecule complexes were established by theoptimisation of all the inter- and intramolecular pa-rameters for the given type of the complex symmetry

Ž .in the RHF restricted Hartree–Fock approximationin basis set HIII. The structures obtained are notnecessarily the global minima because the method ofoptimisation was rather limited. The less stable localminima are omitted in this paper. Basis set superpo-sition errors of the chemical shift were estimated by

w xthe counterpoise correction 21 .Only one of the investigated binary complexes,

water–acetylene complex, is small enough to permitthe evaluation of the solvent-induced changes of theacetylene coupling constants by the supermolecularmethod on the MCSCF level. In this calculationŽ .8 3 3 0 CAS active space is used, containing all thevalence orbitals of C H and the occupied orbitals2 2

of H O other than two lowest s orbitals. These two2

orbitals are inactive as well as the inner-shell orbitalsof C atoms in C H . In this calculation the SCF2 2

optimized geometry was used. The most time-con-suming spin–dipole term of the coupling constantswas not evaluated.

3. Results

3.1. Coupling constants

3.1.1. Coupling constants of gaseous acetylenePrior to the investigation of the environmental

influence on coupling constants in acetylene, thebasis set and the correlation effects on the couplingconstants of the isolated molecule in the gas phase

Ž .were calculated Table 1 . The isotropic couplingŽ .constants Table 1 should be compared with the

acetylene coupling constants measured in the gas

Table 1Ž .The acetylene coupling constants in Hz in relation to active space dimension and basis set

CAS 1 CAS 2 CAS 3 CAS 4

HIII HIV HIII HIV HIII HIV HIII HIV1J iso 191.40 187.41 191.19 187.45 184.41 180.42 191.70 187.70CC1J aniso 36.54 36.28 40.07 39.74 43.30 42.81 36.59 36.28CC

DSO 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01PSO 5.26 5.34 5.31 5.32 5.78 5.78 5.27 5.35SD 8.21 8.58 8.53 8.86 8.44 8.77 8.22 8.59FC 177.92 173.48 177.34 173.26 170.17 165.85 178.20 173.75

1J iso 242.14 238.13 241.43 237.39 242.01 238.22 242.52 238.50CH1J aniso y65.70 y65.45 y66.28 y66.11 y66.30 y65.96 y65.74 y65.47CH

DSO 0.34 0.32 0.34 0.33 0.34 0.32 0.33 0.32PSO y0.59 y0.47 y0.75 y0.62 y0.92 y0.79 y0.59 y0.47SD 0.43 0.40 0.38 0.35 0.41 0.38 0.43 0.40FC 241.97 237.87 241.46 237.34 242.18 238.31 242.35 238.25

2J iso 47.52 47.00 49.85 49.47 49.26 48.81 47.58 47.02CH2J aniso 30.80 30.66 30.40 30.18 31.15 30.94 30.85 30.70CH

DSO y1.34 y1.35 y1.34 y1.35 y1.34 y1.35 y1.34 y1.35PSO 5.06 5.08 5.25 5.26 5.40 5.41 5.07 5.09SD 0.66 0.94 0.76 1.05 0.74 1.03 0.66 0.94FC 43.14 42.32 45.18 44.51 44.46 43.72 43.19 42.33

3J iso 12.33 12.04 12.62 12.43 12.62 12.33 12.36 12.07HH3J aniso 1.39 1.89 1.84 2.30 2.23 2.67 1.38 1.88HH

DSO y3.58 y3.59 y3.58 y3.59 y3.58 y3.59 y3.58 y3.59PSO 4.62 4.65 4.69 4.73 4.71 4.75 4.62 4.65SD 0.54 0.60 0.56 0.63 0.55 0.61 0.54 0.60FC 10.76 10.38 10.95 10.66 10.94 10.56 10.78 10.40


phase because, as will be shown later, the influenceof solvent cannot be neglected. The coupling con-stants of gaseous acetylene in a mixture with xenonare: 1J s174.7 Hz, 1J s247.5 Hz, 2J s50.1CC CH CH

3 w xHz, J s9.4 Hz 5 .HH

The proper correlation treatment is essential forthe accuracy of coupling constants calculations. Theinactive inner-shell orbitals of the C atom do notimpair much the calculated coupling constants, butadding virtual orbitals to active space can improvetheir quality very significantly, which is visible byintercomparing CAS 1 with CAS 2 and CAS 3 re-sults. Both isotropic and anisotropic parts do notundergo monotonic changes with the increasingnumber of active orbitals. It is not possible, there-fore, to conduct any simple estimation of true results.The anisotropies of the coupling constants are partic-ularly sensitive to the active space dimension. Adirect comparison with the experiment is impossiblein this case, because measurements of the tensorialproperties of coupling constants are rare and uncer-

Žw x .tain 22 and refs. therein .To achieve a deeper insight into the coupling

constant properties, the individual contributions tothe isotropic coupling constant were considered, too.

Ž .Not surprisingly, the Fermi contact FC contribu-tion, which usually is the dominant one, is the mostsensitive to the calculation quality. The paramagnetic

Ž .spin–orbit term PSO depends quite strongly on theactive space. The dependence of the most time-con-

Ž .suming spin dipole SD contribution on the correla-tion level, although less eminent, cannot be ne-glected, either. The diamagnetic spin–orbit termŽ .DSO remains more or less constant when the num-ber of active orbitals is changed.

Our results can be compared with the couplingconstants of acetylene calculated recently in a similar

w xRASSCF approximation using basis HIV 22 . Dis-crepancy with the experiment is generally smallerthan that of our results, especially as regards the 1JCC

Ž .coupling constant 181.2 Hz , but with the exception1 Ž . w xof J 232.1 Hz . It should be noted, that in 22CH

the different geometry was used and this factor caninfluence the calculated coupling constants very

w xstrongly 10 .From this part the following conclusion can be

drawn: the calculated coupling constants can be sig-nificantly improved only when a very large number

of active orbitals is used, which requires a largecomputer memory and disc space to be available.The active space CAS 1 containing all valence or-bitals provides sufficient accuracy, not requiring atthe same time an excessive computational effort. Itshould be noted, however, that MCSCF calculationscheme even with fairly extended active space doesnot account for the dynamic correlation effects. Theother source of possible errors is the lack of thevibrational corrections, though using in the calcula-tion the experimental geometry instead of optimisedaccounts to a certain extent for the vibrational effectw x23 .

Extension of the basis set from HIII to HIVintroduces large changes into the isotropic couplingconstant. This is mainly due to the dependence of thedominating FC term on the basis set. The SD andPSO contributions are also strongly basis set-depen-dent. The DSO term, usually the least significant,remains approximately constant when the basis set isextended from HIII to HIV.

3.1.2. Coupling constants of acetylene in solÕentSimilarly as for the coupling constants of the

isolated acetylene molecule, we investigated themethodological aspects of the calculations of their

Ž .solvent-induced changes. In acetone ´s20.7 theseparameters were calculated on various levels of elec-tron correlation. The solvent-induced changes of allthe four coupling constants are governed primarilyby their FC contribution. Only in the case of 1JCC

these changes are significant, so we decided to list in1 Ž .the individual contributions to J only Table 2 .CC

Ž .The resulting values Table 2 depend only slightlyon the active space dimension. This result allowed usto calculate changes of the coupling constants due tosolvent interaction in the full-valence active space,

Ž .with inactive core orbitals CAS 1 , in basis set HIII.Table 3 lists both the experimental and the theo-

retical solvent-induced changes of acetylene cou-pling constants, the latter calculated in the basis HIIIfor the CAS 1 active space, and for the cavity radiusof 6 au. Solvents in Table 3 are ordered according tothe increasing dielectric constant ´ . The theoreticallyestablished sensitivity of 1J to the change of envi-CC

ronment is in excellent agreement with experimentalresults. Also the direction of the changes followingthe increasing dielectric constant is, within a very


Table 2ŽThe solvent-induced changes of acetylene coupling constants in

.Hz calculated in basis HIII in relation to active space dimensionŽ .aR s6 au, ´ s20.7cav

CAS1 CAS2 CAS3 CAS41J iso y3.37 y3.49 y3.58 y3.38CC1J aniso 0.45 0.46 0.43 0.45CC

DSO 0.00 0.00 0.00 0.00PSO y0.21 y0.22 y0.24 y0.21SD y0.10 y0.11 y0.11 y0.10FC y3.06 y3.16 y3.23 y3.07

1J iso 0.11 0.06 0.01 0.11CH1J aniso 0.25 0.28 0.29 0.25CH

2J iso 0.22 0.22 0.20 0.22CH2J aniso y0.13 y0.14 y0.15 y0.13CH

3J iso y0.02 y0.02 y0.03 y0.02HH3J aniso 0.06 0.05 0.05 0.06HH

a The components of the isotropic coupling constant are given for1J only.CC

good approximation, the same. Discrepancy with theexperiment, which is observed for methylene iodide,is attributable to the so-called intermolecular heavyatom effect.

1 ŽVery large experimental changes of J muchCC.larger than predicted by the reaction field method in

case of acetone, acetonitrile and water arise fromstrong hydrogen bond-like interactions of acetylenewith these solvents. They are more clearly visible

Ž .form the plots Fig. 1 of the data in columns 2 andŽ .3, Table 3. The slope of the plot Fig. 1 equal to

Table 3The changes of acetylene coupling constants with respect to the

Ž . agas state in Hz as a function of solvent1 1 2 3Solvent J J J JCC CH CH HH

D D D D D D D Dcal exp cal exp cal exp cal exp

C H y1.36 y1.1 0.05 0.6 0.09 y0.4 y0.01 0.16 12

C H y1.55 y4.1 0.06 0.9 0.10 y0.5 y0.01 0.16 6

CHCl y2.50 y4.0 0.09 1.4 0.16 y0.5 y0.02 0.13

CH I y2.62 y5.5 0.09 1.9 0.17 y1.0 y0.02 0.12 2

C H O y3.14 y5.4 0.10 0.6 0.20 y0.4 y0.02 0.24 10

C H O y3.37 y8.9 0.11 0.8 0.22 y0.2 y0.02 0.23 6

C H N y3.50 y8.3 0.11 1.3 0.23 y0.2 y0.02 0.22 3

H O y3.59 y9.7 0.11 1.7 0.23 y0.4 y0.02 0.32

a w xExperimental values from 5 .

Fig. 1. Correlation of the calculated with the experimental changes1 Ž .of J in Hz .CC

2.245 means that long-range electrostatic interactionsare responsible for approximately one-half of thetotal change of the 1J coupling constant in acety-CC

lene. It is noteworthy, perhaps, that the one-bond CCcoupling constant in acetylene is extremely sensitivenot only to intermolecular interaction, but also to

w xintramolecular substitution 24 .The experimental 1J changes are not repro-CH

duced by the reaction field method. The sign, how-ever, is the same and agrees well with the reported

1 w xtendency of J changes 3 .CH

For the optimized on the SCF level binary acety-lene–water complex the changes of the acetylenecoupling constants were calculated on the CAS level,with BSSE estimated by means of the counterpoisecorrection. The calculated change of the acetylene1J coupling constant is y5.37. The sign is consis-CC

tent with the experimental change y9.7, the size istoo small, but larger than calculated by means of thereaction field method. The analysis of the otheracetylene coupling constants is not straightforward,either because of the destroyed symmetry of thesystem, or because of their very small changes.

The source of the discrepancy with experimentare the approximation of the whole solvation sphereby the one H O molecule, the inaccuracy of the2

binary complex geometry, the limitation of the CASmethod and the lack of the SD term, the first causebeing crucial. Even with these limitations the super-molecular method seems to describe the changes ofthe coupling constants better than the reaction fieldmethod.


3.2. Shielding constants

3.2.1. Shielding constants of gaseous acetyleneThe changes of 13C and 1H shielding constants

calculated for the isolated acetylene molecule withthe extension of basis set and active space are sum-marised in Table 4. Comparison with the experimen-tal data shows the accuracy of calculations of the 1Hshielding to be only slightly impaired by the lack ofcorrelation effects.

The active inner-shell orbitals do not improve theresults of the 13C shielding constants calculationsŽ .compare CAS 1 and CAS 4 results . They undergoconsiderable changes as the number of active virtualorbitals is increased. The calculations on the SCFlevel seem to underestimate the shielding constants,whereas the CASSCF results exceed the experimen-

w xtal ones. A similar tendency has been reported 28 .The extension of the basis from HIII to HIV does notimprove significantly the results.

3.2.2. Shielding constants of acetylene in solÕentTable 5 contains changes in 1H and 13C shielding

caused by interactions with the dielectric mediumŽ .´s20.7 calculated by the SCF and the CASSCFmethods in basis HIII. These results show the depen-dence of the calculated solvent-induced changes ofboth 1H and 13C shielding on correlation level to benegligible. Similarly as for the shielding of the iso-

Table 4The 1H and 13C shielding constants of acetylene calculated by

Ž .various methods in ppm

SCF CAS1 CAS2 CAS3 CAS4 Exp.

Basis HIII:

13 as C 115.89 132.45 129.74 127.89 132.30 117.2

13 bDs C 244.21 220.40 224.40 226.97 220.63 245"20

1 cs H 30.36 30.64 30.56 30.41 30.64 29.3

1 bDs H 15.85 15.57 15.67 15.72 15.57 22"2

Basis HIV:

13 as C 115.71 132.27 129.30 127.70 132.12 117.2

13 bDs C 244.20 220.40 224.78 227.06 220.63 245"20

1 cs H 30.37 30.68 30.58 30.43 30.68 29.3

1 bDs H 15.78 15.47 15.60 15.65 15.47 22"2

a w xExperimental data from 25 .b w xExperimental data from 26 .c w xExperimental data from 27 .

Table 51 13 Ž .Changes of H and C shielding in ppm caused by interaction

Ž .with dielectric medium ´ s20.7 calculated by SCF and CASSCFmethods at basis III

SCF CAS1 CAS2 CAS3 CAS413Ž .D s C y0.55 y0.50 y0.53 y0.55 y0.5013 aŽ .D Ds C 0.91 0.82 0.87 0.90 0.821Ž .D s H y0.20 y0.16 y0.16 y0.17 y0.161 aŽ .D Ds H 0.18 0.15 0.15 0.16 0.15

a Ž 13 . Ž 1 .Symbols D Ds C , D Ds H indicate solvent-induced changesof the shielding constant anisotropy.

lated molecule, the SCF results are close to theCASSCF results obtained with the most extended

Ž .active space CAS 3 .The estimated solvent-induced changes in the 13C

Ž .isotropic shielding Table 6 were obtained by thereaction field and the supermolecular methods. Thevalues labelled ‘SCF’ were obtained by the reactionfield SCF calculation in the gas phase and in adielectric solvent with the fixed experimental gas-phase geometry; the label ‘SCF opt’ refers to thereaction field calculations with the geometry opti-mised on the SCF level for each dielectric constantand values in the next column were obtained also bythe reaction field method in full-valence CASSCFwith fixed geometry. For comparison, Table 6 in-cludes changes of the isotropic shielding constant ofthe acetylene C atom nearest solvent molecule in the

Ž .complex obtained by the supermolecular method13 Žand experimental C gas-to-solution shift with the

.negative sign .Fig. 2 shows the structures of the optimised acety-

lene–solvent molecule binary complexes. The sol-Žvent-induced changes of the shielding constants Ta-

.ble 6 are corrected by the counterpoise method. Theshort intermolecular distances and the large interac-tion energies in the case of complexes with water,acetone and acetonitrile account for considerable sol-vent-induced changes of both 1J coupling constantCCŽ .see Table 3 and shielding constants.

The changes of 13C shielding calculated for thefixed and the optimised geometry differ only slightlyŽ .the latter are slightly more negative , although the13C shielding constants calculated in these two waysare discrepant by ;2 ppm. The observation con-cerning the CASSCF and SCF results is similar, asevident in Fig. 3 which shows the calculated changes


Table 613 Ž .The theoretical and experimental solvent-induced changes of C shielding constant in acetylene in ppm

b aSolvent Exp. Reaction field method Sup. method

SCF SCF opt CASSCF SCF

sys s sys s sys s sys s sysgas gas gas gas gas

c-hexane y3.69 115.67 y0.22 118.13 y0.24 132.25 y0.20 – –benzene y4.45 115.63 y0.25 118.09 y0.27 132.22 y0.23 117.54 y0.75chloroform y5.29 115.48 y0.41 117.92 y0.44 132.08 y0.37 117.69 y0.62methylene iodide y7.97 115.46 y0.42 117.90 y0.47 132.07 y0.38 – –t-butyl alcohol y4.13 115.38 y0.51 117.80 y0.56 131.99 y0.46 – –acetone y5.96 115.34 y0.55 117.76 y0.60 131.95 y0.50 113.86 y4.32acetonitrile y5.81 115.32 y0.57 117.73 y0.63 131.93 y0.52 114.14 y4.05water y6.55 115.30 y0.58 117.72 y0.64 131.92 y0.53 114.71 y3.48gaseous acetylene 115.89 – 118.37 – 132.45 – 115.89 –

a The dielectric constant of the solvent was used in the calculation.b w xExperimental values corrected for bulk susceptibility of the solvent from 5 .

in relation to the dielectric constant: the plots repre-senting the SCF and CASSCF results are just shiftedwith respect to each other.

None of the calculations based on the reactionfield method gave results similar to the experimentalgas-to-solution shifts, as evident from Fig. 4, featur-ing the experimental changes of 13C shielding vs.those calculated by the reaction field method on

Ž .CAS level. The slope of the straight line Fig. 4 isequal to 13.1. This means that, in the case of acety-lene, long-range electrostatic interactions estimatedby the reaction field method account for less than

Fig. 2. The calculated structures of acetylene–solvent moleculebinary complexes.

10% of the overall change of 13C shielding. Points inFig. 4 are rather scattered. The major discrepancyoccurs for methylene iodide, viz., for the 1J cou-CC

pling constant.Even in the present limited extent the supermolec-

ular method seems to work much better for the 13Cgas-to-solution shifts of acetylene than does the reac-tion field method. The results obtained for the binarycomplexes are in qualitative agreement with the ex-perimental values. An exception is the acetylene–be-nzene complex, in which case probably the T-shapedmodel is far from being an adequate description ofthe molecule arrangement in the solution. Discrepan-cies with the experiment are likely to arise mainlyfrom including only one solvent molecule into the

Fig. 3. The calculated solvent-induced changes of 13C shieldingŽ .in ppm in relation to dielectric constant.


Ž .Fig. 4. Correlation of the calculated SCF with the experimental13 Ž .changes of C shielding constant in ppm .

structure of the cluster. We conclude therefore that,at least in the case of acetylene, short-range interac-tions produce the dominant contribution to the gas-to-solution shift.

Anisotropies of the acetylene 13C shielding con-stants and their changes in the solvents consideredare presented in Table 7. Comparison with the exper-imental values is impossible, because only isotropicshielding is observed in the solution. The changes inthe shielding anisotropy obtained by the supermolec-ular method are several times as large as are thoseobtained by the reaction field method. This supportsour conclusion that long-range electrostatic interac-tion do not contribute significantly to the solvent-in-duced shielding change.

4. Conclusions

Coupling constants and shielding constants ofacetylene in the gas phase and the solutions werecalculated. Two alternative methods were employed:the reaction field method, on SCF and MCSCF leveland the supermolecular method on SCF level. Theeffects of correlation and basis set were considered.

The results can be summarised as follows:The calculated coupling constants of the isolated

molecule are strongly dependent on both the basisset dimension and the number of active orbitals. Thecalculated 13C shielding constant in the gas phasedoes not change considerably with the extension ofthe basis set, but depends on the active space dimen-sion. The calculated 1H shielding constant is notstrongly influenced by any of these two factors.

The solvent-induced changes of acetylene NMRparameters calculated by the reaction field methodare not very sensitive to the correlation level. The13C gas-to-solution chemical shifts are very close,whether calculated by the SCF or the CASSCFmethods.

A comparison of the calculated changes of theacetylene coupling constants with the experimentshows the reaction field method to describe to alarge extent the influence of the environment on thecoupling constants. Long-range electrostatic interac-tions are thus inferred contribute considerably tothese changes, although the principal importance ofthe short-range interactions is visible from the inves-tigation of the C H –H O complex. An abnormally2 2 2

Table 713 Ž .The solvent-induced changes of C shielding anisotropy in acetylene in ppm calculated in the SCF and CAS full-valence approximations

Solvent Reaction field method Sup. method

SCF SCF opt CASSCF SCF

Ds DsyDs Ds DsyDs Ds DsyDs Ds DsyDsgas gas gas gas

c-hexane 244.58 0.37 242.09 0.37 220.73 0.33 – –benzene 244.63 0.42 242.14 0.42 220.78 0.38 248.98 7.20chloroform 244.89 0.68 242.40 0.69 221.01 0.61 240.42 y1.36methylene iodide 244.92 0.71 242.43 0.72 221.04 0.63 – –t-butyl alcohol 245.06 0.85 242.58 0.86 221.17 0.76 – –acetone 245.12 0.91 242.65 0.93 221.22 0.82 249.14 7.23acetonitrile 245.16 0.95 242.68 0.97 221.26 0.85 249.73 7.83water 245.18 0.97 242.71 0.99 221.28 0.87 251.22 6.67gaseous acetylene 244.21 – 241.72 – 220.40 – 241.72 –


high change of 1J is evident in both the experi-CC

mental and the theoretical results.The 13C gas-to-solution chemical shifts of acety-

lene are rather poorly described by the reaction fieldmethod, even when the geometry parameters areoptimised for each dielectric constant. The super-molecular method restricted to 1:1 complexes per-forms much better, but even then the calculatedchanges of 13C isotropic shielding constant aresmaller than the experimental. The solvent-inducedchanges of 13C shielding anisotropy calculated by thereaction field method is much larger than the calcu-lated by the supermolecular method. These factssuggest that the contribution of the long-range elec-trostatic interactions to the change of the shieldingconstant observed in the solution is much smallerthan the term resulting from short-range specificinteractions, even for nonpolar non-hydrogen-bond-ing molecules like that of acetylene.

Acknowledgements

We are most grateful to Dr. Michał Jaszunski and´Professor Krystyna Kamienska-Trela for helpful dis-´cussions. It is a pleasure to express our appreciationto Dr. Kenneth Ruud for providing us with theDALTON program. This work was supported by theBST-562r20r97 Grant.

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solvent effects on nmr spectrum of acetylene calculated by ab initio methods

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