Inertia defects of urea

91

Journal of Molecular Structure, 142 (1986) 91-92 Elsevier Science Publishers B.V., Amsterdam -Printed

INERTIA DEFECTS

OF UREA

M.M. CAMPOS-VALLETEl, 'Chemistry

A. TORO-LABBEl,

Dept., University

2Academia

in The Netherlands

Superior

R. CONTRERAS

of Chile,

de Ciencias

Casilla

Pedag6gicas

R.l and G. DIAZ F.2

653 Santiago

Casilla

(Chile)

34-V Valparaiso

(Chile)

ABSTRACT By means of an iterative consistency method it has been determined a complete potential field for the urea molecule. The force constants set which interpretates well the vibrational spectra of urea and its deutero derivatives has been used to evaluate rovibrational constants. The calculated inertia defects should confirm a non-planar structure for urea in gaseous phase.

INTRODUCTION It has been generally responsable

assumed

for its planarity;

real structure

factors matrix

however,

of this molecule.

(ref.1) point out a planar

that II electron

molecule

studies

conformation.

tentatively

assigned

the infrared

possible,

at present,

defect of -0.4262 spectrum

spectra

to confirm

structure

study

On the other hand,

the non-planarity

frequency

of the NH2 group. Since,

that assignment.

for

below 300 cm-l, it is not

Furthermore,

by Brown et al. (ref.3)

by these authors

of the

band at 227 cm-l; this band was

have not been recorded

uma l2 obtained

has been considered

non-planar

conclusion.

absorption

to the inversion

urea-d4,

about the

diffraction

In our opinion, the used temperature

(ref.2) have suggested

on the basis of an observed

for urea is

there exist controversies

In solid state a neutron

are too high to reach a definitive isolation

delocalization

the inertial

from microwave

as a consequence

of a slightly

of urea.

In order to give a further insight on the structure of urea we have performed a normal

coordinate

their vibrational

analysis

and centrifugal

nal analysis

was carried

CALCULATIONS

AND RESULTS

The Normal

Coordinate

force constants consistency

coupling

the total inertia defect,

contributions

out by considering

treatment

(available

method which

The Coriolis

to calculate

and

The vibratio-

a planar C2, syrmnetry.

was carried out on the basis of a GVFF. The

from the authors) were refined

has been tested on several constants,

which

by the matrix

110~ the sum rules,

and they are very sensitive

equation

through

polyatomic

are not inserted

ty, were calculated

0022-2860/86/$03.50

(Avib and Acent).

Atotal,

an iterative

molecules

(ref.4).

for the sakeofbrevi-

of Meal and Polo. These constantsfoto modifications

0 1986 Elsevier Science Publishers B.V.

of the force

92 field. The vibrational the formulation according

inertia defect of 6.74 x 10m4 uma g2 was calculated

of Jeyapandian

the equation

FinaA!y,

following

by using

et al.(ref.!S). The value of Acent evaluated

of Oka et al.(ref.6)

Herschbach

was found to be 5.17 x 10v4 uma !*.

et al.(ref.7)

we have obtained

a value of 0.078

uma A’ for Atotal. DISCUSSION The potential spectra

an excellent

of urea and their deutero

tble values; A/rad*) energy

field provides

in particular

function

charges

describing

interactions,

of the vibrational

The force constants

derivatives.

that concerning

since it is in agreement

reproduction

the torsional

with that value obtained

the whole

surface

the steric hindrance

and composed

effect

have reason-

normal mode (0.105 mdyn by means of a potential of three terms;

and the electronic

the net

contribu-

tion, the last one being the most important. The positive

value of Avib is as expected

data (ref.5). The Coriolis vibrations

make a definitely

compensates

the positive

vibrations.

This situation

Concerning

contribution causes

to Atotal

the values

the inertia

negative

the Acent value,

this contribution

Since

interactions

defect,

according

between

contribution

to Avib, which

from the interaction

the small magnitude

it has been reported

is small and positive

Of Avib and Acent are small, Aelec,

to experimental

nearly

among the in-plane

of the calculated

that in several molecules

the

electronic

seems to be the main contribution

The procedure good results appears

potential

followed

correction

to be reliable

concluded

w*) and the experimental about the non-planarity

inertia

(ref.8).

and consistent

that the difference

on

to Atotal. contribu-

function.

to evaluate

in other molecules

avib.

(ref.5).

The high value of Aelec is associated with the important electronic tion to the torsional

reported

the out of plane and in-plane

between

one, -0.4262

defects

in this work has given

Thus, the value of Atotal

with a planar model. the calculated uma k)* (ref.3),

of the urea molecule

obtalned

In this sense,

value of Atotal

here

it is

(0.078 uma

could be a new evidence

in gaseous

phase.

REFERENCES J.Worsham, H.Levy and S,Peterson, Acta Crystallogr., lO(1957) 319-323. S.T.King, Spectrochimica Acta, 28A (1972) 165-175. R.D.Brown, P.D.Godfrey and J.Storey, J,Mol. Spectrosc., 58(1975) 445-450. N.Mercau, R.Aroca, E.A.Robinson, J.Aron, J.Bunnell and T.A.Ford, J. ComPutational Chem., 5(1984) 427-434. S.Jeyapandian and G.A.Savari Raj. J. Mol. Structure, 8(1971) 97-105. T.Oka and Y.Morino, J.Mol. Spectrosc., 8(1962) 9-21. D.R.Herschbach and V.W.Laurie, J.Chem. Phys., 35(1961) 458-463. R.Aroca, J.Anacona and E.Clavijo, J. Mol. Structure, 27(1975) 49-53.