MO investigations on lignin model compounds VIII. A PCILO study of intramolecular hydrogen bond in guaiacol and o-vanillin

315

Advances in Molecular Relaxation and Interaction Processes, 14 (1978)315-320 0 Elsevier Scientific Publishing Company,Amsterdam-Printed in The Netherlands

MO INVESTIGATIONS VIII, AND

A PCILO

ON LIGNIN

STUDY

MODEL

COMPOUNDS

OF INTRAMOLECULAR

HYDROGEN

BOND

IN GUAIACOL

0-VANILLIN+

REMKo++

MILAN

Pulp

and

Paper

Research

Institute,

890 20 Bratislava,

Czechoslovakia (Received

29 June

1978)

ABSTRACT The

semiempirical

chemistry phenol

and

energy

The hydrogen

with

of the benzene is 6.07

stable

ring

equal

O-H...0

a planar

five

is membered

of the formyl

a nonplanar

group

energy

in 2-methoxy-

in guaiacol

oxygen

and fonms

of the -CHO

stereo-

(o-vanillin).

bond

and forms

to 15O. The

to study

phenol

by carbonyl

of rotation

used

bond

the hydrogen

fonmed

is more

an angle

been

hydrogen

of 2.7 kJ/mole bond

in o-vanillin

gTOUp

ring

has

2-methoxy-6-fonmyl

to my calculations,

and has

ring.

method

of the intramolecular

(guaiacol)

According weak

PCILO

six membered

out-of-the-plane

of this hydrogen

bond

kJ/mole.

INTRODUCTION The

guaiacol

end group

of the guaiacol and

inter-

the semi-empirical

+FOT

PaTt

Scientific University,

VII

structural

units

Cll. The ability of guaiacol to form intrahydrogen bonds has been thoroughly studied by

methods

of guaiacol

++Present

to the basic

lignins

molecular

experimental bonds

belongs

[2-41,

and

The

inter-

its derivatives

CNDO/2

see Tef.

theoretical

L81

and were

intraalso

method

molecular

studied

hydrogen

[6-81 by

[5].

l

address: Research

Institute,

Kalinciakova

ul.,

Department Bratislava,

of Pharmacy, Czechoslovakia

Komensky

316 In this paper influence

the results

of the rotation

the intramolecular The

for a hydrogen

bond

conjugated The

energy

formation

PCILO

method

surfaces

angles

of the hydrogen

to the minimum

conformer

trans

forming

with

orientated

the intramolecular

groups

studied

, v' and

bond

study

have

been

8 (Fig.1).

the difference

on energy

surface)

-OH group bond

and

Torsion

The

angles

calculations

at the Computer the QCPE

experimental

RESULTS The

AND

geometry

guaiacol

are

performed

determined For

the

in energy

between

the

the cis conformer

is taken,

studied

on a Siemens

of the Komensky For

of several

4004/150

University,

the theoretical

computer

Bratislava

calculations

using

the

[18] was used.

DISCUSSION

results

indicated

Centre

220 programme.

used

2b

in the compounds

were

on

o-vanillin

successfully

2a

1.

zero.

recently

of the

and

conformational

p

hydrogen

1 Fig.

[g] was and

and fopmy

in guaiacol

in the compounds

(corresponding

calculations

methoxy energy

rlo-13 3 [14-17).

of the torsion

energy

bond

study

mqlecules

by variation

of both

hydrogen

are presented.

of the PCILO

of the PCILO

shown

with

The most

on Fig.

respect stable

calculations 2. Isoenergy

to the global conformer

(Fig.

of the energy curves

energy

surface

in kJ/mole

minimum

taken

2) is that with

for

are as energy

an intra-

60

30 I 190

-30

-60

-90 -30

Fig.

2. PCIM

molecular

bond

group

60

conformational

hydrogen

2.7 kJ/mole) this

30

0

and forms

situated

of the hydrogen group,

that

introduced

the CNDO/2

calculations

located

stability with

Substitution hydroxyl bond

the guaiacol concerned hydrogen

c24 with the

bonds

formed

to the O-H...0

l

Further

ring.

through

stereochemistry

in o-vanillin.

method. have

pairs

position

strong

considerably studies

3

Fig.2)

of the

are

Similarly shown

that

symmetrically

axis.

in the ortho can form

pair

in OCH

approximation

dimers

the lone

acceptor

(p =O",

one lone

the CNDO/2

(energy

The hydrogen

plane

to the PCILO

when

is weak

of the proton

of the phenol

kg]

which

membered

pairs

bond

integrals

for

-COCHg)

-CHO,

hydrogen

210

guaiacol

by

of guaiacol

by a group

(-N02,

five

lone

for

be caused

is achieved

respect

surface

180

150

to the molecular

bond

could

originally

greatest

both

symmetrically

instead

energy

a planar

with

methoxy

120

O-H... 0. This

bond

interacts

90

by

intramolecular

influences the PCILO

of both

to the phenolic hydrogen

the acidity method

0-H...O=C

were

and

of therefore

0-H...OCH

3

318

E n e r g y surfaces

of o - v a n i l l i n are r e p o r t e d in Fig.

surfaces were

o b t a i n e d b y the v a r i a t i o n

an~les

and ~ (Fig.

an~le

8, ~

~

I)o In the case of c o n f o r m e r

a n d for c o n f o r m e r

energy values

3 and 4. E n e r g y

of the t o r s i o n 2a the

2b the 8/~le ~ were h e l d at 0°o The

are c a l c u l a t e d

with respect

to a b s o l u t e

minimum values

90

60

30

0

-30

-60

-90 -30

0

30

60

Fig. 3. PCILO conformational o-vanillin

and expressed

in kJ/mole.

hydrogen bond O-H...O=C w i t h h y d r o g e n bond. ~Toup

twisted

to

of this h y d r o g e n

90

120

energy surface

According

type c o n f o r m e r

180

2a of

3 is m o s t

stable

and forms a six m e m b e r e d

~ =15 ° out of the b e n z e n e

(Fig.

4) w i t h

rin4~ plane.

as the d i f f e r e n c e

h y d r o g e n b o n d e n e r ~ y in o - v a n i l l i n

2'10

for conformer

to the Fig.

This r i n g is n o n p l a n a r

bend calculated

'150

and ~ / a i a c o l

the rin~

the CHO

The ener6~y

between

represents

the 6.07 kJ/mole.

319

160

120

h\\-23’/

/

/-

\\

0 90

60 /19

\

/15\

30

\ \

0

-30 -30

0

30

60

90

120

150 a

2p

180

Fig. 4. PCILO conformational energy surface for confomer o-vanillin

TABLE 1 PCILO net studied Conformer

0

of

-

-0.2059

1 2a 2b

-0.1926 -0.2427

The in

charges

net

compounds

charges studied

O-H...

H

bonded

. ..OCH.

0.1301 0.1283 0.1562

of

0 hydrogen

the are

atoms

in

the

210

2h of

molecules

. ..o=c

-0.1625 -0.1600 -0.1~6

atoms listed

forming in

Table

the 1.

hydrogen Substitution

bond

O-H...0 of

the

CHO

320 group

in the ortho

the acidity comparison

of the hydrogen with

more

stable

than

that

position

that

hydrogen

to the phenolic

hydroxyl

bonded

in the conformer

in guaiacol. bond

hydrogen

On the other

O-H... O=C

(conformer

hand,

group

decreases 2a by

hydrogen

2b) ie more

of the

acidic

in guaiacol.

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