Gasphase conformational analysis of dimethylsulphite

451

Jou~fofMolecuPrStruchtre,79(1982)461-454 Elsetier ScientXic Publishing Company, Amsterdam

GASPHASE

CONFORMATIONAL

R. L. ODEURS,

ANALYSIS

Printed in The Netherlands

-

OF DIMETHYLSULPHITE

B. J. VAN DER VEKEN and M. A. HERMAN,

RUCA, Groenenborgerlaan

171, 2020 Antwerp

(Belgium)

and P. VAN NUFFEL, UIA, Universiteitsplein

1, 2610 Wilrijk

(Belgium)

ABSTRACT The newly recorded conformationally A quantitative FORTRAN

gasphase

infrared spectrum of the title compound

based on the known existence profile

IV-program

simulation

of the (S=O)-stretching

for use on a mini-computer,

of the two conformers

Outof

present.

of two conformers

tures, we propose the conformers

mode,

is applied

using a self-written

to deduce the structure

a total of six theoretically

present

in the gasphase

is analysed

in the liquid phase.

possible

struc-

to be (pro-R +sc,pro-S

+sc)

and (pro-R +sc.pro-SLap).

INTRODUCTION The existence

of two conformers

has been well established with a conformational

(refs. l-3).

analysis

Besides,

towards two totally deuterated

plication

different analogue

tions. duct.

of dimethylsulphite

data have been reported,

electron

(ref.

infrared

in the one region which technique,

relevant

together

(ref. 4), but

for quantitative

diffraction

structures

the gasphase

(DMS)

experiment

pro-

pointed

In an attempt

5).

spectrum

of DMS and its

seemed most relevant

namely the (S=O)-stretching

for apregion.

DATA

The spectra were recorded absorbance

phase

in those regions

confonnational

of a third independent

EXPERIMENTAL

Gasphase

a recent gasphase

to settle this problem we rerecorded totally

liquid

based on dipole moment measurements

the spectra were rather undetailed file analysis.

in the

on a Perkin-Elmer

scale, thus allowing

580 infrared

for direct comparison

spectrometer,

of experiment

on an

and calcula-

In figure 1 the v(S=O) is shown for both the normal and the deuterated A minor feature

upon deuteration. -1 CID in the nonal

on the low frequency

It is assigned

side in the nonnal compound

to a CH3D-rocking mode , which

product and shifts to approximately

is found near 1180

900 cm-l after deuterating

the product. 0022-2860/82/0060+000/$02.75

pro-

dissappears

0 1982 Elsevier Scientific Publishing Company

462

1200

1160

Experimental

Fig. 1.

GEOMETRICAL

lz50

cm”

u(S=O) for DYS (upper trace) and DMS-d6

AND INERTIAL PARAMETERS

Considering

that eclipsed

each prochiral

rotamers are energetically

O-atom can have its CHS-group

namely one trans and two gauche projections amounts

to a total of nine combinations,

In Klyne-Prelog

notation,

under

these are the combinations and

the

(+ap.+ap).

INDO-methods ,

mental

gasphase

approximating system.

is

(+sc,fap),

of 60°.

This

before pro-S,

assigned

(-sc,Lap),

These geometries

(-SC,+=) configurations,

were then optimized

fit of the structures

in the experi-

Using these refined geometries,

data (ref. 5).

observed

some a priori-information frequency

and the

vibrational

from the Morse wavefunctions, excited

new value for r(S=O) inertial were calculated.

anharenergy

mean dissociation

potential

and a deviation

level relative

constants

regarding

(ref. 7), thus

mode v(S=O) in DMS with a diatomic

values for the mean displacement

for the first vibrationally molecule

pro-R

(ref. 6) were fit into a Morse-type

the molecular

The expectation

calculated

that

(+sc,+sc),

for obtaining

in this mode, the here

of an (S=O)-bond

positions,

AO, BO and CO were calculated.

As a trial calculation monicity

for DMS,

of which only six differ energetically.

were deduced from these staggered

electron diffraction

inertial constants

stand in three different

by a least squares

and

most unfavorable

relative to the (S=O)-bond.

assumption

(+sc,-SC),

Start geometries

setting torsion angles to a multiple

by using

(lower trace).

from equilibrium of about'O.01

(S=O)were then

A was found

to the ground state.

Al, BI and Cl for a vibrationally

With this excited

453 The amount of pure A-, B- and C-type simple projection axes.

this mode as pure and perfectly

with inertial

constants

for asymmetric

derived

be communicated

The separation

one of the profiles the band maximum. high frequency

30 cm -1 .

with a full width at half maximum information

for the two conformers intensity

over

it can be safely assumed

of the higher frequency

u(S=O)-profile

As all theoretically

experimentally

u(S=O)-profiles

To simulate

area-normalized

triangular

this second convolution

conformer,

namely

be discarded

whatsoever.

are gathered

(+sc,+sc).

For the high frequency

then was performed

Therefore

between models

but the experimental sterical

hindrance

experimental

to

spectra.

almost exactly

component models band structure.

based on models

Found that

4 and 5 should A weighted

1, 3 and 6.

As a measure

of theory from

for model 1 this sum was

3 and 6, for which the goodness-factor

we rejected model

profile

shows no pronounced by model

3.

1.

It is not possible

P- and R-branches,

Also in this geometry

between the two methyl-groups

6 and assign the high frequency

fits

assign this band to the second

3 and 6 on the basis of least squares analysis

which is only nearly fulfilled reject model

of

squares of the deviations

; it was

about two times as much as for models distinguish

(which are difficult

the

We therefore

of the fit, the weighted

the same.

nor

in figure 2.

band.

were summed and compared

approximately

it appears

interactions,

profile for the second conformer

low frequency

least squares analysis experiment

in the two 0-CH3 groups

since they do not show the appropriate

of the goodness

that

simulated profiles again with an -1 function with a half-width of 4 cm . The results after

It shows that the calculated the non-structured

we can conclude

do show POR-structure,

is too simple for exact reproduction

these influences we convoluted

side

and vice versa for the

Therefore

profile.

shows no PQR-structure

predicted

of internal rotations

observed

that the low frequency

that the used model, which does not include vibration-rotation a priori),

Of 2 cm-l,

on the program will

have indicated that not -1 30cm to the left or right of

by the profile due to the other conformer,

the low frequency

IV-program

The model calculations

has considerable Therefore

side

FORTRAN

(ref. 8).

of the band maxima

to approximately

conformers

In this case the broadening function used

line was Gaussian

elsewhere

is not perturbed

using a self-written

J-value was 100 ; more detailed

while the maximum

influences

localized.

for each of the six possible

as explained,

top profile simulation.

for each individual

determine

by

ANALYSIS

Pure A-, B- and C-types were calculated

amounts

was calculated

of a unit vector along the (S=O)-bond on each of the inertial

This implies that we approximate

QUANTITATIVE

in the u(S=O)-profile

than in model

component

6.

was to

alone,

a condition

there is much less We therefore

to the w(S=O) of conformer

3.

464

6

(\ -40

Fig.

40 -40

Simulated

2.

40 -40

v(S=O)

for

each

&-I

of the

4b

six

theoretically

proposed

conformers.

CONCLUSION Quantitative sulphite and

profile

is to a vast

simulation majority

has

present

established as two

that

distinct

in the

gasphase

conformers,

dimethyl-

namely

(+sc.+sc)

(+sc,ap).

ACKNOWLEDGEMENTS Professor ject.

The

extensive

H. J. Geise

(UIA)

"Studiecentrum

voor

computer

is thanked Kernenergie",

for

stimulating SCK-CEN

Mol,

discussions Belgium,

on the

sub-

is thanked

for

facilities.

REFERENCES 1 J. C. Lavalley and 0. Saur, J. Chim. Phys., 69(1972)1149. 23(1974)81-91. 2 P. V. Huong and E. Raducanu, J. Mol. Structure, 3 A. 8. Remizov, A. I. Fishman and J. S. Pominov, Spectrochim. Acta 35A(1979)901-907. 4 L. K. Yuldasheva, A. P. Timosheva, A. B. Remizov, G. N. Sergeeva, A. I. Fishman and A. N. Vereschagin, Bull. Acad. SC. Ser. Chim., 23(1974)289. 5 P. Van Nuffel and H. J. Geise, to be published. 6 P. C. Weast (Ed.), Handbook of Chemistry and Physics, 59th Edition 1978-1979, CRC Press, p. F 229. 7 P. M. Morse, Phys. Rev., 34(1929)57. 8 R. L. Odeurs, B. J. Van der Veken and M. A. Herman, to be published.