Far infrared spectra of methyl nitrate and methyl-d3 nitrate

Journal of Molecular Structure, 142 (1986) 105-110 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

FAR INFRARED SPECTRA OF METHYL NITRATE AND METHYL-d3

B. J. van der Vekenl, G. A. Goirgis*,

105

NITRATE

and J. R. Durig*

'Laboratorium voor Anorganische Scheikunde, Rijksuniversitair Antwerpen, Groenenborgerlaan 171, B 2020 Antwerpen (Belgium) *Department (USA)

of Chemistry,

University

of South Carolina,

Centrum

Columbia,

SC 29208

ABSTRACT The far infrared spectra from 300 to 50 cm-l of methyl nitrate, CHsONO,, and methyl-d, nitrate, CD,ONO,, have been recorded at a resolution of 0.12 cm-l. The fundamental methyl torsional mode has been observed at 204.5 cm-l (154.2 cm-l for CD,ONO,) with two excited states falling to lower frequencies which gives a V, barrier of 980 + 40 cm-l (2.80 f 0.11 kcal/mol). The NO, torsion (methoxy) has been observed with the 1 + 0 transition being at 133.7 cm-l (119.5 cm-l for CO,ONO,) and eight successive excited states falling to lower frequencies. From these data the twofold barrier to internal rotation has been calculated to be 2650 + 75 cm-l (7.69 + 0.21 kcal/mol).

INTRODUCTION A rather

complete

Cawthon

(ref.1)

sions.

However,

rotating

based

as well

as

From

later

a

(methoxy) ments of

able

to

cm-l

Raman

CH,

determine

study

states (9100

the

torsional

+

study nor

116 of NO, the

a resolution

values

(ref.2),

a

that

and

excited

matrix

cal/mol). barrier

state

excited There

has

been

infrared

spectra

of

cm-'.

0.12

which

These

for

internal

been

material directly

The

and

NO,

(ref.2)

using

torsion

were

also

of the methyl these

a barrier

observed

of 812 + 41

recent

but apparently Therefore

and CD,ONO,

vibraneither we

have

from 300 to 50

of this study

0 1986 Elsevier Science Publishers B.V.

entropy

intensity measure-

rotation

observed.

results

is freely

band contours.

the

a relatively

(ref.3)

of CH,ONO,

the two tor-

gives a twofold barrier

authors

herein.

0022-2860/66/$03.50

infrared

they obtained

also

except

and experimental

the

splittings

state

isolated have

to

by Brand and

the NO, group

frequency

of the NO, torsion

threefold

torsions far

assigned

proposed

of the statistical

f 2600

cal/mol). the

being

was published

to be 130 + 20 cm- l from relative

from the second

investigated at

cm-l

modes

depolarization

microwave

of CHsONO,

(ref.1)

on the comparison

f 909

(2321

tional

authors

was estimated

splittings

study

all the normal

these

the

from

group

cm-l

with

of the excited

3182

the

vibrational

are reported

106 EXPERIMENTAL The sample nitric

of CHsONO,

acid

deuterated

in

the

species

(0.12

cm-')

material.

vacuum

far

prepared of

was prepared

used as the starting low temperature

was

presence

similarly

except

spectra

column.

of the

helium

polyethylene

cooled

Ge

windows.

bolometer

Both 6.25

and the sample was contained

CDsOH

equipped

containing

and 12.5p

in a lmeter

of methanol

acid

by sublimation

Mylar

The was on a

high resolution

recorded

from 300 to 50

with a vacuum

a wedged

with

(ref.4).

(98% deuteration)

The relatively

gases were

cm-l on a Nicolet model 8000 interferometer liquid

sulfuric

Both samples were purified

fractionation

infrared

by the esterification

concentrated

sapphire

beamsplitters

bench and a filter

were

and

employed

cell.

RESULTS AND DISCUSSION infrared

spectrum

of gaseous

Fig. 1B and the corresponding

The

observed

spectrum

of methyl-d,

the "light"

far

molecule

the 1 + 0 transition

200

WAVENUMBER

Fig. 1. methyl-d,

Far infrared nitrate.

spectra

of

(A)

nitrate

nitrate

for the methyl

is shown

in Fig.

1C.

group is observed

in For at

100

125

150

175

methyl

(cm-‘)

water,

(B)

methyl

nitrate,

and

(C)

107

204.5

cm-l with

have

been

two well

assigned

corresponding strongest

the

Irrespective

beginning

of which

energy

(V,/2)(1

- cos

constant,

F =

internal threefold

first

set

other

First,

0.02 kcal/mol)

was the

the

calculated. calculation

and

an

F value

with

deuteration. is consistent

molecule.

Utilizing

oscillation

reduced moment

as:

moment of

carried

out

the

V(d)

=

of inertia

inertia

The calculations was

for the

were using

done

the in

only the

but the transitions

was carried

were fit

out using the first three

fit.

Similar

of 2.9528

respectively.

as the

was

of

two transitions

this

cm-l gave

carried

out

*60 cm-l, and positive

molecule.

transitions

since

it

the

to be

than

the

the

CD,ONO, (2.83 f

as the 1 + 0 and 2

the 3 + 2 transition

was

at

transitions

at

only 134.38

cm-l

calculations

and

gave a The

fit to &5 cm-'. and when

to 968 f 40.cm-l

Further

160.11

which

it is

but the last

using a Vs term

the fit but the value was rather large,

is inconsistent

Therefore,

for

respectively,

were

is lowered

we

is due to greater

is heavier

have frequencies believe

which

improved

The

in Table 1 is from this calculation. using

are only fit to *3 cm-l.

latter transitions

light

appears

the barrier

1.

bands was

a V, of 989 f 7 cm-l

From this calculation

pair

in Table

was off by 3 cm-l although

calculations

1 +- 0 and 2 + 1 transitions,

transition

with these

is shown

and the fit using all four observed

using the 154.22 and 147.98 cm-l transitions

to the calculation

kcal/mol),

is a set

194.11cm-l and only a V, term which gave

calculation

better

added

We

and

reduced

since the third transition

were

third

rotor

6a)

is the

There

a value which

torsional

of 989 + 60 cm-l but the transitions

the

factor

the fit of the transitions

calculation

cm-l

torsional

molecule.

shift

obtains

at 139.3 cm-l and the fit given

further

for

- cos Ir

at 204.49

better

+ 1 transitions,

barrier

one

a threefold

was predicted

three

using

predicted

since the

with both V, and Vs terms which gave values of V, = 1018 f. 19 cm-l

transition

142.86

for

The second

not significantly

A

appropriate

is used,

(V,/2)(1

and Vs = -24 f 9 cm-l;

molecule

The

respectively. assigned

the next one at 148.0 cm-l which

of 970 f 19 cm-l (2.77 + 0.05 kcal/mol)

transitions

the

in the "light"

cm-l with

the

to rotation

ways.

to only 2 cm-l.

fourth

2 transitions,

from the data for the CH,ONO,

+

two transitions

a barrier

3 +

194.1 and 178.1 cm-l which

and F has a value of 5.546 cm-l for the light molecule,

barrier

different

torsion

h2/8n21r,where

rotation

states at

at 160.1 and 142.9 cm-l have too large a separa-

and

function 3o)

1 and

at 154.2

spacing

the value obtained

potential

two

2 +

to the methyl

correct

excited

for the CD, group are not readily

set of transitions

of transitions

with

the

transitions

tion compared

has

as

defined

CH,

with the value for this parameter

believe

the

coupling rotor

complexity

of this

of

rotor with

and the torsional

the

CD,

the NO,

transitions

closer to those for the NO, top. the

which

methyl is the

barrier average

has a value between

those

of 980 f 40 cm-l obtained

(2.80 + 0.11

for the CH, and CD,

108 TABLE 1 Far infrared rotations

torsional

of methyl

transitions

due to the CH, or CD, and NO, internal

nitrate.

CH,ONO, rel. u(cm-l)

int.a

204.49 196.20 194.11 192.22 188.61 178.08 160.00 139.21 136.82 136.52 134.99 134.58 133.74 129.64 125.96 124.61 124.28 122.39 118.92 115.48 112.43 109.59 109.21

b

I+0

S

CD,ONO, v(cm-l)

Ad

assignment

0.71

&,w VW

-1.90 -2.71

3+2 4+3

W

VW

assignment

171.64 166.32 160.11 154.22 151.31 147.98 142.86

-0.34

VW 2+1

rel. inta

1 +

Ad

07

I+0 b

-0.92

2+1 2 f l?

0.25

3~2

0.66

m VW VW W

C

S

I+0

S

2+1 3+2

0.93 0.06 -0.33

4+3 5+4 6+5 7~6 8~7

-0.53 -0.56 -0.47 0.10 0.98

S

VW W S S

m W

VW VW

sh VW

142.74 140.31 137.73 137.41 135.61 134.38 131.78 129.00 124.93 122.68 119.91 115.84 113.84 111.75 110.82 109.42 107.63 104.38 103.12 97.62

W

W W W

m VW VW VW

3 + 2?

W

m m

1+-o

C

0.97

2+1

0.18

3~2

-0.53

4+3 5+4 6~5 7~6

-1.18 -1.04 1.62 -0.02

W

m VW W

m W W VW

aAbbreviations used: s, strong; m, medium; w, weak; v, very; sh, shoulder; bbr, broad. The transitions for the CH,(CD,) torsional transitions are from nearly triply degenerate (A,E) energy levels but are indicated by single digits. 'The energy levels for the Nfz rotor are essentially doubly degenerate (AlrAz) dbut they are indicated by single digits. The A stands for the observed minus the calculated values.

rotors

using the first

obtained

for the

considerably

expected

utilizing

study

study

(ref.2),

(ref.2)

difference

the

where

than the value

rotor and the second ever,

transitions,

light molecule

higher

from the microwave largest

three

estimated

we calculated

than the value we obtained

a V6 term was

structural

was estimated

an F number

the splittings parameters of 5.784

with the determined

This value is

(2.32 kcal/mol)

the Io in the ground

state where

spans the value

utilized.

of 812 f 41 cm-l

where the uncertainty

between

excited

and the uncertainty

from the

state of the methyl were

listed

observed.

How-

in the microwave

cm-' which

structural

obtained

is 4.3%

parameters

larger

(ref.5).

Considering the

two

both

possible

values

are

coupling

between

barriers

would

assignment

and

whereas

one

would

1.3796

cm-l

which

is

begin

3.5 at

series

for CHsONO,

were

in

which

carried

believe

two

out

difficult, closely

lowest

by energy

good agreement a frequency

gives

for

levels

reasonable

CDsONO,

the

with

fit to the transitions

barrier fore, more

although the

by adding

CH,

molecule believe

calculated

reflect

the

so we

values

the actual

and

NO,

from

gave

methyl

barrier

sponding

barrier

nitrate

and nitrite

of

the

cm-l

light

these

molecule.

values

with

an

should

be the

least per-

This value is in remarkably study (ref.2). where if one corrects

the

may

is about 2.4 cm-l larger than it

is possible

obtain

large V, term.

is shown

in Table

give

an

is reasonable two

1.

a

For

Similar

effect

the

(Table

spacings

1).

so

There-

is expected

Possible

low

to

coupling

a calculation

be very instructive.

is quite

in cis-methyl

artificially

transitions

for the NO, rotor.

the C-O bond distance

to

V, = 1957 ? 21 cm-l and V, =

results

first

of 980 f 40 cm-l 669

of

a V, term 400 cm-l lower and a V,

these

barrier

rotors

Similar

an F number

the

a relatively

are:

carried out with such coupling would probably The

well

barrier,

the fit of the transitions

barrier

nearly

between

for

between

transitions

this

the fit of the transitions

larger

as the

data (ref.5).

f 8 cm-l where

cm-l

cm-l.

with

from the microwave

201

for the CD,ONO,

to obtain

of 2690 f 27 cm-l

to k1.2

molecule

normal modes.

the determined

100

fit

value

light molecule

calculations

from

for the light

In order

barrier

are

in the potential

from other

rotor

barrier

particularly

well.

is somewhere

of the observed

NO,

CH,ONO,,

and only a V, term with an F number

the

term

twofold

of 130 + 20 cm-l was used for the NO, torsion,

the

for

i.e., 2650 + 75 cm-l (7.69 f 0.21 kcal/mol),

with the value obtained

for

rather

is 2609 + 33 cm-l (7.46 f 0.09 kcal/mol) with

F number by using the new structural

expected

is

the 2 + 1 at 129.64

frequency

the

a periodic

the

barrier

frequencies

Since the spacing

that the

spaced rather than diverging

transitions

agreement

the

transition

in

of

periodic

barrier

to span these values,

the

is

values would differ

cm-l,

lower

Calculation

twofold

the

reasonable

we

uncertainty

there

119.91cm-l with the first few "hot bands"

become

a normal

torsional

at 133.74

cm- 1

is rather

the transitions

and the twofold

Therefore,

turbed

about

these

kcal/mol);

calculations

that

but it would be surprising

(methoxy)

0 falling

in frequency.

cm-'

f 0.07

since

ones

for

lead to a 10% error,

is possible

that the barrier

we used the first two transitions

1.6752

(7.69

NO,

1 +

lower

found

barrier, of

of

since

be

the

for CO,ONO,

cm-l

compound

to the extent

could

It

by these two techniques.

the

successive

4

either

of

with

those

about

of error which

in agreement.

the CH, and NO, torsions

straightforward cm-l

not

be effected

by the amounts determined The

sources

still

high compared

nitrite

(ref.6).

to the correFor oboth the

has been found to be 1.437 A and the

110 N-O(CH,) 1.398

bond distances

i,

the CH,ON moieties the PCON

which

nitrite.

Thus,

barriers

other

with

it is difficult nitrate

to rationalize

to compare

barriers

in other

even

barrier

larger.

for

the same except for

nitrate.

However,

the PO'NO

angle in cis-methyl

the differences

nitrite

the methyl CH,OX

in the methyl

on the basis

barrier

molecules.

in the nitrate.

For

example,

the

of structural

in the nitrate

The methyl

with

barrier

in

There are other CH,-0 barriers

barrier

is 1070 + 52 cm-l (ref.8) and the methyl

is 1339 f 70 cm-l (refs.9,lO)

Finally,

it

should

which

hot bands originating in an excited it difficult of

be

intermingled

torsions

therefore

which clearly

in

barrier

indicates

methyl

hypochlorite,

in methyl

vinyl ether

that the "X" group has a

effect on the methyl barriers of the CH,OX molecules.

transitions

some

the

parameters

ether has been found (ref.7) to be 942.6 ? 8.7 cm-l which agrees well

pronounced

nitro

are essentially

for

and cis-methyl

interesting

methyl

are

CH,OCl,

smaller

structural

is 3.3“ larger than the corresponding

the corresponding

which

degrees

with values of 1.402 i and

the

between the two molecules.

It is also

dimethyl

different

Therefore,

for these two compounds

in methyl

variations

(refs.5,6).

is two

(cis) in the nitrate -

some

are only slightly

respectively

mentioned

that

with the assigned

have not been assigned. from excited

to ascribe bands

be

a

large

explained

number

by

of weak

for both the methyl and

Some of these bands are probably bending

They appear as "clumps"

them to a particular

could

are

states of the low frequency

state of the other torsion.

these

there

transitions

normal mode. coupling

mode or

which makes

It is possible

of the

two

that

rotors

and

further studies along these lines are needed.

ACKNOWLEDGMENT The authors the

NATO

gram,

gratefully

Scientific

Grant

NO.

acknowledge

Affairs

569-82,

Division

and

by

the

the financial through

support

of this

the collaborative

National

Science

study

research

Foundation

by

by

proGrant

CHE-83-11279.

REFERENCES 1 2 3

J. W. J. S. J.

C. Cl. Brand and T. M. Cawthon, J. Am. Chem. Sot., 77 (1955) 319-323. B. Dixon and E. B. Wilson, Jr., J. Chem. Phys., 35 (1961) 191-198. A. Lannon. L. E. Harris. F. 0. Verderame, W. G. Thomas, E. A. Lucia and Koniers, J: Mol. Struct.,-50 (1974) 68-81. 4 R. Johnson (Ed.), Organic Synthesis, Vol. XIX, John Wiley and Sons, Inc., New York, 1939, p. 64. 5 A. P. Cox and S. Waring, Trans. Faraday Sot., 67 (1971) 3441-3450. 6 W. D. Gwinn, R. J. Anderson and 0. Stelman, Bull. Am. Phys. Sot., 13 (1968) 831. 7 P. Groner and J. R. Durig, J. Chem. Phys., 66 (1977) 1856-1874. 8 J. 5. Rigden and S. S. Butcher, J. Chem. Phys., 40 (1964) 2109-2114. 9 P. Cahill, L. P. Gold and N. Owen, J. Chem. Phys., 48 (1968) 1620-1626. 10 J. R. Ourig and 0. A. C. Compton, J. Chem. Phys., 69 (1978) 2028-2035.