α-Substituent effects in 13C NMR of hydrocarbons : Quantum chemical calculations

α-Substituent effects in 13C NMR of hydrocarbons : Quantum

chemical calculations

Citation for published version (APA):

van Dommelen, M. E., Buck, H. M., & Haan, de, J. W. (1978). α-Substituent effects in 13C NMR of hydrocarbons

: Quantum chemical calculations. Chemical Physics Letters, 57(1), 80-82.

https://doi.org/10.1016/0009-2614%2878%2980355-8, https://doi.org/10.1016/0009-2614(78)80355-8

DOI:

10.1016/0009-2614%2878%2980355-8

10.1016/0009-2614(78)80355-8

Document status and date:

Published: 01/01/1978

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VoIume 57, number 1 CHEMICAL PHYSICS LETTERS 1 July lP78

Q-!SIJBST~TUEIW EFFZCTS IN 13C NMR OF HYDROCARBONS. QUANTL-S CX-iEMICU CALCULATiONS

Marianne E_ VAN DOMMELEN, Her& M. BUCK and Jan W. DE HAAN

Lubomtones of Organic CTzenuMy axd hstrurne~ta~ Ana~yszk, Eindhoven Uinive?si~ of Tedww~ogy,

Ein&orven, The ffetherhnds

Received 21 Clc:obzr 1977

The “C NMR u-substitr;cnt effects within the series methane to neopentane acd ethene to isobutene have been calculat- ed rrdng Hartree-Foc?c perturbation theory with XlINDO/3. The sign and order of magnitude of the calculated a-effects are in agcemen: with new experimental -&dues, including effects of multipie substitution.

Recently, the 13C chemical shifts of small hydrocarbons have been measured in the gas phase [I ] _ The result- ing a-substituent effects, without disturbing differences in solvent effects, are given in table 1. The largest discrep- ancy with the previous experimental a-effects [2] is the difference between methane and e*&ane: 14.1 ppm in- stead of 8.0 ppm_ The new creffects show that the consequences of multiple r _ninal methyl substitution become progressively smaller, analogous to the well-known downfield shifts of C2 in ~ .e series butane, 2-methylbutane and 2,2slimethylbutane, measured as neat Liquids [2] _ In table 1 a survey is given of a-substituent effects from known theoretical 1% shift calculations [3-71. It should be realized, however, that not all of these calculations

were aimed explicitly on the effects of multiple methyl substitution. The difference between methane and ethane can be reascnabIy reproduced in sign and order of magnitude with a finite perturbation, a coupled Hartree-Fock* or a Pcple method. Serious discrepancies between calculations and experiments, however, occur whenever effects of multiple substitution are considered. CalcuIations with the Pople method [6,7] do not reproduce the observed decrease of the o-effects within the series methane to neopentane. The difference between ethane and C2 of pro- pane caiculated by means of finite Perturbation calculations is equal to or larger than the difference between methane and ethane, in contrast with the experiments_ Sometimes, C, of propane is calculated even upfield from ethane. E!lis [4] ascribes this to the set of iNDO parameters used or the basic approach employed_ The shielding results of Ditchfield et al_ f3] are dependent on the origin of the vector potential describing the magnetic field, which is taken at the center of mass.

We c&cuiated *SC shifts with the concept of Strong-et al. [P] _ Only the second order paramagnetic term was calculated. The expression for the xx component of the screening tensor of atom A is given by (eq. (16) of

Strong et al_ ]9]>:

alI

occ cnocc atoms

C C F & (CiyAcjZA-c~AcjyA) (RyA II/r: IRZA)

i i

* o-ctTects, uicuhtcd by coupled Hartree-Fock perturbation theory [S-IO] were not incinded in table 1, because only the shift difrerence between metkane arld ethane is avvaikzble fcr comparison with experimental values.

Table

1

Expcrimcntal

and

calculated

WUbStitUCnt

cffccta

(positive

numbers

rciatc

to

downfield

shifts

in

ppm)

C *C

c-9

c-c-*cc

c-*c-c

c-4-c

c=*c

c=*c-c

c_*c’+_c

Rcf,

C C

**c-c

-+

c-v-c

-t

c-c-*c-c

I

+

c-v-c

I

-+

c-“c-c

3

c=*c-c

3

c=*c-c

3

c-“c”-c

1:

::

1:

8.00)

IO,2

14.1

11,8

13.2

18,s

23.8

25.5

15,6

23,4

1,5

-1.7

3,o

-0,3

1.7

7.1

4.0

3.4

13,7

13.4

14.2

12,4

3.6

3.4

-23.3

-9.4

9*4

₁₀₄₈

9.1

7.8

2,l

3.2

10.3

12.7

11.3

25.3

27.3

9.5

11.2

9.9

8.1

9,2

15.7

13,8

4.8

S,O

-120.2

-2,2

14.6

b)

4

4

4

0

f;‘,

0

0

k)

0

no

n)

14,l

12s

3.5

-2.6

13,6

6,s

4.4

8,9

15.0

9*7

25,3

a)

A

Shift

difference

of

11,2

ppm,

ob;aincd

by

Pugmirc,

iS

also

quolcd

by

Strong

et

al.

[9],

The

expcrhncntal

conditions

arc,

howcvcr,

not

given.

The

commonly

accepted

value

was

8.0

ppm,

b)

Expcrlmcntnl

[ 21. 4

Bxpcrimcnkl

from

gas

phase

mcnsuromcnts

[

11,

4

Calculnted;

finite

pcrturbntlon

method

with

ab

initio

with

diffcrcnt

basis

sets

[

31,

up,

(The

use

of

UP

+

Ud

results

m

upfield

a-zffccts,)

LIZMAO.SG

standard

scnlc,

0)

As

in

d),

but

with

LEMAO-SG

optimized

scale,

3

As

in

d),

but

with

4-31G.

lJ

Calculnted;

finite

perturbation

method

with

INDO

[4],

up. 11)

Cnlcuiatcd;

finite

perturbation

method

with

INDO

[4

]

,

up + Ud , 1)

Cnlculnted;

finite

perturbation

method

with

INDO

[S] , up + U& 1)

Calculated;

Popic

method

with

a

constant

AE

q

10

eV

with

an

LCAO

MO

theory

[6],

up, k)

Calculated;

Popic

method

with

B

constant

AE

=

10

cV

with

ENT

[

71,

up, 1)

Appronch

(l),

see

text.

m)

Approach

(2),

see

text,,,

n)

Approach

(3),

see

text.

vcalme 57, riumlxr 1 CHEbmAL PHYSICS r.JzrTERs X July 1978

(The effective nu&ar charges were calculated without a @orrection parameter, i.e. fl= 1, eq. (35) of

Strong et al.

:9] .)

We used the

MMDO~3 procedure fl I]

_ All geometries were optimized with MINilO/3. For the energy differ-

ence between a ground and an excited state molecular wavefunction, three approaches were used: (0 mz+j = IO eV, without invoking the closure rule.

(2) A.&$ = 4Ci - pi>, tic: energy difference between an occupied MOi and an unoccupied MOi. (3) AEV = -(Ei - ei> + Xii -

Jii, the

singlet transition energy.

Results of

calculated aeffects are given in table i

t .

Approach (21 shows remarkably good results. The sign and order of magnitude of the crdculated a-effects is in ageement with the experimental values, inchrding the effects of multiple substitution. We realize that approach (I) can only yield approximate results. In this concept refmements of one single aspect like AEV are not warranted as can be seen from the results of approaches (2) and (3). For calculation cf psubstituent effects approach (1) is

inadequate. A!l &effects ca&ulated were smaller than 1.5 ppm. This could be due to the

neglect of two-center one-

electron terms (eq. (17) of Strong et al. l9] ). PresnrnabIy approach (I) is also applicable for calculating tu-substi-

tuent effects in heterosubstituted hydrocarbons.

The authors wish to thank Theo RoSeIs for vah.tabIe discussions. This investigation has been supported by the Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).

Referaces

Cl 1 L..LM. vm de Ven and 3-W. de Haan, J. Chern. SOC. aem. Coxnmun., to be published; unpublished results. $21 J.B. Stotbers. carbon-13 N!&R spectroscopy (Academic PXCSS, New York, 1972);

G.A. Ok& and DJ. f)onoxm, J. Am. Cbem. Sot. 99 (1977) 5026. [3f R. Ditchfield, D.P. Xifier snd 1-A. Pople, 3. Chem. Phys. 54 (1971) 4X%. {S] P.D. EIlis, G.E. Maciel and 1-W. bfciver, J. Arm Cbem. SOC. 94 (1972) 4069. [Sl M_ ICondo, I. Ando, R. CEjja and A. Miioka, J. bfagn. R-n. 24 (1976) 315. [6] T. Yonezawa, I. Mori@ima and H. Kate, Bull. &em_ Sot. Japan 39 (1966) 1398.

[7] V.N. SoIkan, V.M. Mkmayev, 5Lb$ Sergeyev and Yu. A. Ustynyuk, Org. Magn. Resow 3 (2971) 567. iSI H. Katb H. &to and T. Yonezawa, BufL Cfxem. Sot. Japan 43 (1970) 1921.

191 A.B. Strong, 5. Ikenberry and D.M. Grant, 3. Xxg~ Reson. 9 (1973) 24.5. [lOI R. DitchfieId, Mot Pkys. 27 (19i4) 789.