2

179

Advances in Molecular Relaxation and Intemction Processes, 19 (1981) 179-194 Elsevier Scientific Publishing Company, Amsterdam - Printed in Belgium

METHANOL-PYRIDINE RELATION ENERGY

J.P.

OH

DETERMINED

SEGUIN,

80039

de

BONDED

FREQUENCIES

COMPLEXES

AND

THE

: COR-

n ORBITAL

CND0/2

UZAN chide

organique

Amiens-ceder,

J.P.

HYDROGEN

STRETCHING

BY

R.

Laboratoire

physique,

Universite’

de

Picardie,

France

DOUCET

Institut

de

versitd 1,

DERIVATIVES

BETWEEN

de

rue

et

VII,

de

Brosse,

Guy

(Received

Topologie

Paris la

de

Dynamique

Associd

au

75005

des

SystBmes

de

Z’Vni-

C. N.R.S. Paris

, France

14 March 1980)

ABSTRACT

The electronic and the energy 17 pyridine method. thanol

derivatives

between

compound.

the n orbital linearly

fact

we observe

ionization

a different

that the substituents

withdrawing

using

There

the CNDCl/Z

parameter

methods

are electron

of me-

of the

characterizing

show that AvOH

are

; with CNDO/Z

potentials

sensitivity

atom for

is no correlation

of energies

by different

with

orbitals

of OH stretching

and a charge

The comparison

obtained

correlated

calculated

pyridines.

of AwOH

on the nitrogen

molecular

the weakening

to these

the variation

pyridinic

localized occupied

have been

We have measured associated

results

population

of the highest

depending

donating

on the

or electron

ones.

INTRODUCTION

Despite of pyridines been recently to release subetituents cations

numerous with

studies

a variety

a renewal

their

of interest

electrons

as shown

are generally

these

last years

of proton

donors

on the complexation

[l-27],

on the aptitude

or to be reorganized

in many

papers

[27-361.

devoted

to the study

there

has

of these

depending The first

of different

0378-4487/81/0000-0000/$02.50 0 1981 Elsevier Scientific Publishing Company

bases

on their publiparameters

180 characterizing authors

this

use

variations through

of

of

par'ameter

the

ionization the

be

generally

with

ST0

been

very

the

3G

basis

water-pyridine molecules

Coming bonding the

ciation

of

lating

of

as

AvoH

charge

be

with

the

more

a

orbitals

semi-theore-

bulkiness

for

by

initid'approach.

of

hydrogen to

study, if

variously

these we

donor

involved

"ab

[36,39]

of

Furthermore the

bigger

this

to

model bases

evaluate the

along

the

asso-

substituted

electrostatic

parameter

has

like

calculation

than

17

of

but

molecules

intended

of an

one

calculations

studies is

series

from

The

molecules

derivatives

with

charge

most

size

handle

work

benzene

with

of

contrary

general

described a

to

their

[37].

the

small

has

pyridines

with

semi-empirical

present

calculations.

of

On

our

transferred

energies

medium

easier

methanol can

CNDO/Z

for

to

and

departure

SCF

the

OH

method of

or

on

initio"

used

are

for

w-HMO

atom

depending

of

pyridines

theoretical

"ab

[36].

weakening

the

of

between

correlated

with

more

been

the

lines

pyridines

from

after

[36-401,

same

been

phenol-pyridine,

[14-16,291

the

various

series

use

extended

complex like

methods

has

recently

of

the

one

relationship

to

some

obtained

only

by

nitrogen

First

results

analogous

the

and

correlate

evaluated

has

choice

molecules.

with

linear

associated from

approaches,

associated

an

protonated

electrons

works,

tical

For

potentials

n

recent

density

[13].

to

A

to

concerning

associated

IT charge

ease

parameters

characteristic

methanol

[l-13,28]

derivatives

association.

established

their

association

approaches

AuOH

of

nitrogen

been

of

experimental

the

stretching

of

pyridine

semi-empirical

partners the

type

numerous

as the

acceptor

correderived quantity and

the

transfer.

EXPERIMENTAL

In the association disagree

on

complex

the free II.

a number

methanol

values.

between [table

literature, of

For

value and

with

of

instance.

AvOH

we

just

have

have

OH

varies

measured

been

presented

compounds, for

characteristic

associated

Therefore

of results pyridinic

the

of from the

the 266

AuoH

but

on the

authors

methanol-pyridine difference to

307

for

17

cm

-1

associa-

181 TABLE AvOH

1 values

for

associations

No.

substituent

1

H

266

2

z-me

3

3-Me

4

of

methanol

with

literature data

"OH

substituted

pyridines

references

266

-

307

4,7,11,13,1,2

309

304

-

312

11.13

296

301

4-Me

296

265

5

2,4-!JiMe

330

332

13

6

3,5-GiMe

312

316

13

13 -

305

7,4,11,13

7

2,6-DiMe

332

325

6

2,5-DiMe

332

332

9

3,4-DiMe

315

307

-

315

11.13

10

2,4,6-TriMe

344

346

-

356

13.1

11

3-CN

200

12

4-CN

211

13

3-Cl

230

14

3,5-DiCl

180

15

2-F

162

16

2-OMe

230

17

4-NMe2

362

“OH

= ‘OH

accurate

tions

and

to

of

standard 0.2

[free1 +_ 3 cm

methanol

M for

checked

molecular ride

[quality

by sieves

164

GPC. and

Uvasoll

13

13

-

172

11.13

4

352

measured

[associated)

pyridine

derivatives,

: temperature are

11.13

in

CC14,

are

.

with

methanol

333

241

‘OH

-1

conditions

Pyridines been

-

-

and

pyridines

commercial The

also

using

respectively

has under been

the

following

concentrations

products,

methanol

maintained has

3O’C.

and been

[table their

distilled,

argon. dried

0.02

Carbon on

molecular

M

11.

purity

has

dried tetrachlosieves.

on

182 Infrared

spectra

were

run

(working

in

beam

model

cells.

For of

band, is

the

weaker

are

ciations,

RESULTS

energy

bonded

bution

the

variations

OH

This

approach

has

120'

the

methyl

of

286,298 -1 cm for of 5,1423

e

new

band

asso-

180

cm

-1

,

to

using

the

Pople

120'

and

then

in

109'

increase

by

pyridine

the

of

well the

e.

For

5,1329

the

on

substitutions

is

as

on

molecular

groups

nitrogen to

positions the

electron

(qil

3,5-dimethyl 2.4

electronic populations

and

decreases

and

derived

1).

Thus,

AvOH

an

values

increase the

of

electronic decreases

qN the

elec-

atom

the

z

along

which

methyl

populations

substi-

compounds,

also

by

possi-

derivative,

while

pyridine. 6

for

placing

direction:

same

nitrogen

two

one

[table

showing

the

atom,

the

the

pyridine,

plane

angle

progressive

group,

these

[311.

calculation.

opposite

in

has [421.

standard

the

amino

with

alkyl

methyl

atom

for is

the

regularly

cycle

valence

the

ring

correct

Gordon

stable For

be

the

performed

Pople

parameters

and

is

atom

of

pyridinic

more

nitrogen.

nitrogen

to

pyridine

the

localized

from

distri-

method

al's

demonstrated

2-Me0

supplementary

to

IT charge

as

et

near

nitrogen

population

thermore,

been

the

Bak

the

CND0/2

pyridine and 3,5-diMe -1 312 cm respectively.

each

the

orthogonal

using

shows

pyridine

and

tron

AvOH

of

charge

on

the

3-Me

from

the

OH

Measureof

range

electronic

derivatives

calculation

values

the

all

For

and

AvOH

are 12

this

;

group. series

a

localized

described

the

J

group

pyridine,

For

recently

anglesof

charge

one

whole

over

infrasil associated

cyano

the

using

geometry

1431.

conformations

for

population

are

ble

tutions

For

second

main

the

and

calculated

fixed

subtituents

The

the

extends

stretching

[41]. a

parametrizations

used

a

spectrometer

path

derivatives

been

Bevaridge given

we

is

of

281

optical

DISCUSSION

has

is

cm

with -1

+-3cm.

to

been

oxygen

that

accurate AW

1 there

than

electronic

21

Perkin-Elmer

with

complexation

pyridine

The

The

a

methanolic

in

[table and

to

AND

a

on

cyanopyridines,

attributed

ments

OH

dual

axis

represents

but On

more the

groups

the

sharply contrary, increase

qN

and

q i.

by

the

CNDO/Z

As,

furmethod

183 TABLE

2

Electron

populations

of substituted

compound No.

,experimental

and calculated

,(a) 9N

9N

44

5.1423

1.5229

1.0579

lllb) ST0 36

!J exp.

2,12

2,06

2,22cc1

I,05

581646

I,5176

1.0863

I,95

3

5.1369

1.5271

1.0447

2.25

4

5,153o

1.5198

1.0766

2,40

5

5.1737

1.5143

1.1042

2.21

6

5.1329

I,5305

1.0303

2.34

7

5.1044

1.5107

1.1136

1.03

0

5.1564

I,5211

I,0717

2,03

9

5.1495

I,5239

1.0636

2.49

IO

5.1954

1.5095

1.1325

2.01

11

5.1386

1.5230

1.0521

2.92

12

5.1375

1.5225

1.0527

0.91

13

!?,I374

1.5160

1.0610

2,26

14

5,1304

1.5108

1.0624

0,16

15

5,1797

1.5225

I,1115

3.35

IfiStrans

5.1665

1.5231

I,1299

3.62

16Scis

5,2016

1,5165

1.1319

I,53

17

[I201

5.1803

I,5133

I,1231

3.15

17

(109)

5.1757

I,5148

1.1146

2.76

x axis

going

(b)

values

of

ref.

(31)

(cl

values

of

ref.

(441

Cd)

values

of

ref.

(33)

give

consistent

experimental no

through

the

results ones,

correlation

which

moments

CN;O,2

2

la)

dipole

pyridines

conclusion

for

(table

Av

between is

in

nitrogen

the

21,

it

and OH contrast

2,37

the with

2.53")

2,03")

1.34

2,70

and

be

dipole

atoms

moments

concluded

TT charge the

C-4

the

calculated must

l,95[c1 2,3BCc1

on

results

that the of

and there

nitrogen Perkampus

is atom. [131.

184

The in

x

electron

axis

atoms)

q

of

with

pyridine of

much

account

the

carbon

ween

the

give

satisfactory

:

meter

more

the

charge

on

of

cycle,

a

non

value

methanol

of

the

complex

(266

charge

association

of

small

been

calculated

by

theoretical

that

this

approach

pyridine-HF

and

computering

time.

only To

the

the

metry

of

method

a

the

145,

whereas

methanol,

of in

On the

the

atom the

7

the

second

COH

first

was

the

is

charge

para-

charge

of been

the

the the

pyridines

[45]

complexes.

on

the

geo-

optimization

the

N,H,O OH

on

calculations

depends

throughout

al

in

charge

CNDO/Z

parameters

to

like

[14-16,24,25.29].

transferred

nl

has

noticed

increase

use

the

transfer be

alcool-pyridine

the

and

imstrength.

systems

the the

of

(named

pyridine-

relatively

larger

we

used

the

must

used

charge

Inamed

105,2”

in

charge it

to

has

one ring

one

of

repre-

be

a

a

despite

geometric

perpendicular angle

bond

methanol-substituted

461

not

kind

association

and

extended

transferred

pyridinic

is

the

associations

the

complex,

Rinaldi

plane

been

influence

the

and

certaintly

molecules,

methods

of

OH

for

associations,

that

this

electronic

the

127.361

for

the

the

series

fact

models.

nitrogen

But,

AvOH

methods

pyridine-H20

clear of

of

ciation the

make

for

Given

size

now

on bet-

complexes

account

semi-empirical

strength

method

has

that

does

for cannot

and quantitative

to

as

pyridines.

weakening of -1 3 implies cm

transfer

taking

well

atoms,

that,

between

Furthermore,

distances

contribution

associated

bond

as

ring out

in methanol-pyridines

The

type.

comes

variation

e.

different

pyrldinic

It

relationship

for

hydrogen

transferred

portant

the

the

4-CN

for

1391,

atom

to

very

the -4

correlation

and

related

AvOH

4.10

C-4

varies

pyridins,

nitrogen

electrostatic

single

of

and

results.

calculated

Strength

the

the

the

be

it

while

only

atom

and

cannot

for is

sophisticated

atom

bond, by

that X qN

of

nitrogen

because

instance,

than

the

nitrogen

parameter

for

but

the

on

the

structure,

values

hydrogen

sented

For

with

smaller

atoms

hydrogen

through

This

Av,,~

corresponding

tentative,

into

localized

going

substituents

is

the

axis

heratically.

variation

little

a

;

varies

the

population

[x

for

two

OH

bond

atoms

bond,

cycle

of

and

the

are

assois

In

aligned,

centered the methyl

baee.

on For group

the the

185 TABLE

3

Geometric

parameters

and total

energy

for

methanol-pyridine

complex

py-MeOH

rOH(h

rNH(b

rCo(h

total energy

n

model

1,056fa'

1.514(b)

1,369

49661.1

A

model

1.053

1,595

1,369"'

49660,s

(a)

ltterature

value

1 A" (25)

(b)

ttterature

value

1,8

rCO

(cl

is

not

optimiaed

of

transferred

A” in

(Kcal/mol)

(24)

7i model

TABLE 4 CNDO/Z values pyridines

complexes

x lo4

Aq

617

H

for

methanol-substituted

e

pyr idine

xsubstituent

charge

MeOH

Aqz

Aq’y

Aqz

Aq’y

0

-617

-169

609

3,5-OlMe

625

62

-707

-192

616

4-Me

630

53

-663

-193

622 655

2,4,6-TriMe

649

237

-006

-207

3-CN

600

0

-600

-181

702

4-CN

605

0

-605

-165

769

2-F

595

0

-595

-201

796

for methanot is

Aq

-

:

the

monomer chapge

electronic

F

co

=

1,371

transferred Zoss

j

+

i,

from :

FOH

=

1,032

pyridine

electron

uptake

to

A"

methanol

186 carbon

atom

plane N

is

the

perpendicular

atoms,

the

to

new

small

variations

ready

found

cribable

to

"ab

initio"

for

the

ferred

of

the

CNOO/Z

n

from

total

the

the

out

of

the

since

total

the

zation

in

to

the

acceptor.

ved

plane

exists

It

this

the

The

therefore

as

the charge

out 4

the

the

plane

it

can

be

the

since

the

to

energy

the

in

a

acceptor,

0,

the

reorganiH

atoms.

a

tendency bond

donor a

only

and

observed,

hydrogen

for

the

larger

charge

are

the

look

of

Is

a

molecule

C,

the

out

methylated

which

that

from

the

,

charge

H,

of

results

density

by

inferred

[Aql

the with

transfer

for

methanol

strength

our

electron

phenomena

of

than

charge

plane,

results

advisable

and

a

kcal trans-

substituted

compensated

of

the

charge

contrary,

reverse

transferred

AvOH

and

to

the

simpler

rela-

orbital

methanol

involis

al-

or

being electron

the

energy

pyridine

of the highest

AvOH

molecular

Koopmans's

the

is

in is

of

an 5

1.11,12,15

the

the

Concerning

between

between

M.O.

calculations

[IPI

[Aqxyl charge,

table

charge

on

On

as-

consistent

compound

effect 0).

alis

about

and

is

The

complex,

larger is

model

there

3.

by

methanol

complex

For

no =

used,

of

n

case,

loss

plane.

transfer,

Relationship

by

qz

twice

the

[26]

same

amount

which

Table

same.

occupied

since

has

and

in

between

by

ways

the

the

IT complex,

the

HF

stability

contrary,

modi-

complexes,

and the

O.H,

little

in n

a

the

only

II and

for

higher

methanol.

(A

of

in

containing

pyridine

for

lies

collected

since, a

it

suffer

are

of

every

transferred

the

quantity

tionship

to

pyridlne

correlate

the

use

In

charge

results

and

energy

when

approximately

the

transferred

whichever

From

4.

plane

total

increase

is

transfer

derivatives,

ring

energies

Therefore,

pyridines

pyridine

tha

the

we

table

electron

the

than

On

results.

in

of and

method,

in

complexes,

shown

I

whole

complex

are

ring

shows

transferred

pyridines

the

association

1271.

preceding

of

the

the

the

of

parameters

for

form

in

ql

that

distances

calculation

n

quantity

[A

plane

Optimization

fications.

the

in

orbitals

energies

experimentally theorem

equal

to

transfer

leads that

of

[47].

can

be

by

photoelectron

to

take

the If

the

molecular there

can

determined

by

spectroscopy,

ionization orbital be

no

potential involved

contestation

and

187 about

experimental

bands

of

for

CNDO/Z the

method

is

6,

not

the

carbon

atoms

carbon

with

a strong

atom.

This Bene

31,

and of

the

a

second and

of

distribution in

Immediately type,

then

is

which

the

important

orbital

atom the

is

lower

is

located

sion

located

a 71 orbital

from

n orbital

in

orbital

beneath,

sigma

nitrogen

different

the

of

the

pyridine

strongly

then

the

of

n because

atom.

cycle,

the calcu-

molecular

of

a2

assign

As an example,

orbitals

as

to

sequence of molecular or-

occupied

defined

orbital

easy

semi-empirical

331.

molecular

contribution

[311,

same

[48.

nitrogen

2.3.5.6 atoms

always

the

highest

essentially

a ‘II molecular

the

the

: the

not

Furthermore,

give

describes

on

is

1331.

molecule

order

localization tuated

do

a given

following

[HOMO]

Del

spectra

methods

lation bitals

it

values,

these

of and

one

b,

the

on type

4 carbon

proposed

than

the

by first

IT

orbitals.

al.

In

a study

on

1371

obtained

a

corresponding

Avo,,

to

0

the

the

protonation

linear

of

relationship

highest

pyridines. between

occupied

n orbital.

Ramsay the If

et

pK% and one

IP&

looks

10 \

9

300

OCP b

o\

2 1

.o

250

\

200

0

13

12

0

0

ill-0

IL 9.5

Fig. 1. *'OH

10

\ 15

0

b

IP

Methanol-substituted pyridines complexes : correlation between of the n orbital.

and the ionization potential

188 for

a

correlation

pyridines

energy

in

the

[361,

ring

of

points

withdrawing

with

AVOH

1,

and

result

relative groups.

the

relationshiD

AvDH

between

table

IP

On

of

n

:

(Figure

the

three

the

for

IP

methanol

of

the

is

unsatisfactory,

to

pyridines

the

contrary

electrons

T

associated orbital owing

to

substituted

the

is

fairly

orbitals

of

with

of

the

with

of

and

scatte-

electron

correlation good

the

highest

these

follows

the

1)

TABLE 5. Energies in

of

first

substituted

pyridines

eV

compound No

energy

energy

n

‘n

n

a2

energy bl

1

-12,98

-13.87

-14.45

2

-12,73

-13.00

-14,32

3

-12,79

-12,97

-14.33

4

-12,75

-13.42

-13.85 -13,53

5

-12,49

-12,85

6

-12,61

-12.58

-13,74

7

-12,54

-12,53

-13,89

8

-12,56

-12.25

-14.29

9

-12.56

-12.73

-13.55

10

-12.28

-12.56

-13.08

11

-13.21

-13.27

-14.51

12

-13,26

-13,68

-14,27

13

-13,09

-12.84

-14.81

14

-13.26

-12.89

-13.77

15

-13.34

-13.49

-14.89

16

Strans

-12,74

-12,39

-14.48

16

Scls

-12,93

-12,34

-14,48

17

(1201

-12,83

-13,ao=:

-15.52

17

Cl091

-12.79

-13.79::

-15.04

:: the second

orbital,

the

first is

nitrogen atom of the NMeZ group.

n

essentially

locatited

on

the

189

*%H with

=[- 149

z

AIP = IP

8)AIP

l (287

(X pyridinel

* 4)

- IP(pyridinel

for 11 compounds the C-C. being 0.98 and the mean deviation -1 Bcm . For all these compounds, the values of the potential the n orbital

is comprised

to the two 71 orbitals, pyridines

for which

and its methyl first

between

except

the IP of n type

derivative,

of these

1331 the use of either the validity the n type

difference

eV and 0,lO

potentials

is a matter

two values

of the correlation.

For cyano

even when

their

3-CN.

4-CN

For pyridine

between

the

eV respectively.

of these

IP's.

corresponding

4-Me,

Is the lowest.

the energy

two IP's is only 0.15

the assignment

the IP values

for pyridine,

energies

of

Even

if

of controversy

does not nearly dsrivatives,

modify

we choose

are lower than

those

of 71 type. In table

5. we show the energies

of substituted is seen that

pyridi-nes, calculated the first

which

is -13.87

group

is introduced

a methyl

group

in the 4 position. groups

in the 3 position

group

by about

between

AvOH

0,19

The energy from the most withdrawing relation

eV. Therefore

of the first

electron

ones.

exists

donor

by about

all elec-

: a Cl

there

1 eV and a 4-CN

is no relationship

of the highest n orbital

occupied

varies

subtituents

For the 15 first

between

a methyl

also this orbital

destabilizes

and Au

n orbital.

monotonously

to the most

pyridines

the n orbital

It

of

and only by 0.45 eV for

In the same manner,

destabilize

and the energies

the energy

by 0.9 eV when

in the 3 position

orbitals

the same method.

of pyridine,

eV, is destabilized

tron withdrawing atom

?r orbital

of the molecular

using

electron

in table OH

1, a cor-

and follows

the

relationship

*'OH

= (174

with AE

'_ 1438~~

n = enIX

and the mean

deviation

the 4-NMe2

oxygen

atoms

terizes

'_ 51

; the c. c. is 0.96

pyridinel

lation

ciation,

+[250

- 'ncpyridinel 13 cm -1 . Are excluded

and 2-OMe

pyrldines

of the substituents

involving

a modification

both associations

from this

in which

are possible of Av

corre-

the nitrogen sites

which

OH at the same time,@iLWre

for asso-

then 2)

and

charac-

190

AVOH

300

250

200

2.

Fig.

Methanol-substituted

A more the

linear

two

correlations

precise

the

to and

n orbital

pyridine we

“OH mean For

“OH mean

0,7

eV,

to

a AV

of OH a A’E~

corresponds

substituted

=[92

111

substituted (324

deviation

two

cm

4318~~ 5.6

only

cm

new

electron the

points

In

envelop

of

donating

2,4,6-trimethyl of

the

energy

linear electron

+_

eV.

It

electron

comes

out

withdrawing

:

relationships donating

groups

41

-1 with +(251

-1

0.36 and

+[283

pyridines z

the

ie

a variation

with

AE~

: 5,2

deviation

=

obtain

it

with electron withdrawing -1 between pyridine and cm

124 of

that

slopes. With -1 between cm

donating

pyridines

z

fact

whereas,

electron we

58

OH corresponds

of

separate

substituents, For

of

of

of the distribution in

different

a AV

pyridlne

substituents,

if

shows

of

substituents,

: correlation

complexes

of the n orbital.

analysis

regression,

pyridine

pyridines

and the energy

AvOH

between

.

electron z

191

withdrawing

groups

of

2-F that

191 An interpretation pyridines

toward

tituents

of nitrogen

as derived

a n electron the pyridines the fact orbitals electron confirmed straight having

of pyridines.

withdrawing

substituents

by the higher

larger

than

tends noticed

mentioned.

lation

overestimates

strongly

that

higher of

[49] as

[Fig. 31. The

donating

it can be added

effects

Despite

the effect

is underestimated

their

separating

to stabilize

ceding remark that for this type of substituents, of the energy

correof the

and the IP of

a breaking

already

to electron unity,

relationship

the energy

part of the correlation

line corresponding

of subs-

for all

the homogeneous

shows

been

of

and the IP's of the n

calculation

also

generally

it has already

that,

linear

between

into the two classes

[48],

a slope

AvOH

from a CNDO/Z

that CN001.2 method

on the nature

Furthermore,

of the lone pair

in sensitivity

in mind

is a single

parameter

atom.

for the series

n orbital

depending

if one keeps

there

studied,

the experimental

electron lation

associations

can be proposed

the pyridines between

of the difference

substituents to the prethe calcu-

on the variations

level.

(E;y , -13

-12.5

Fig.

3.

CNDO/Z

Correlation between the energy of the n orbital obtained by method and the ionization potential of the same orbital fo,r

pyridine?. derivatives.

CONCLUSION In type

the

between

the

rizing

the

tment

of

donor

acceptor is

band

is

of

the

of

different

with

the like

methanol, to

related

to

or

those

measured

correlation

ones,

molecular

on

and

All

these

phenol-pyridines

the

of of

CfrOO/Z

linear

the

method,

correlations

with

electron

seems

to

the

point

effects those

CNDO/Z

also

out

of of

of

by

(which

lead

that

the

electron

dona-

electron

the

with-

energy

of

the

n

association.

methanol-pyridines results

complexes

the

li-

abstraction

calculated

variation

by

the

As

stretching

spectra

the

the

of

by

electron-

groups.

underestimates

trends

bonding.

AvOH is

by

two

trea-

AvOH.

the

energies

slopes)

involved

to

single

hydrogen

that

when

photoelectron

concerns

confirmed

shown

the

characte-

between

substituted

overestimates

as

transfer

obtain

orbital

two

the

calculated

pyridines

of

with

orbital

completly

we

any

in

corresponding

But

of

correlation

parameter

corresponding

energy,

no

excluding

have

withdrawing

method

substituants

drawing

for

bonding

is

charge

charge

orbital.

the

electron

comparison

calculation

IP

n

a

the we

orbital,

slopes

The

a

the

the

molecular

donating

and thus

varies

from

hydrogen there

contribution

complexes,

correlated

OH

AvOH

compound,

always

electron

n

of

electrostatic

these

by

pyridines,

pyridinic the

and

nearly one

ting

complexes

variation

For

acceptor

to

association

methanol-substituted

of

a

complexes systematic

are study

of

[501.

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