The determination of π-electron densities in azulene from C13 and H1 nuclear resonance shifts

Tetrahedron Letters in Great Britain.

No.

14,

468-472,

pp.

1961.

Pergamon

THE DETERMINATION OF n-ELECTRON DENSITIES FROM cl3

of

Pure

and

Chemistry,

THERE is

on substituted related

to

the

C7H:

lol:al

the we have

and

C8Hi-,

H1 resonances by Leto, Fig. density, are than

also

the and

relative

and

two

shifts

of monosubstituted as N.R.C. Leto,

the

F.A.

which

and

has

No.

of

molecules.

of

been

3 implies

have

are

some

parallelism

that

the

known.

The

measured Richards.2 n-electron

ref.

2 above

times

larger

the

resonance

of

in

C6H6,

the

fifteen

observed

C5H;,

and

in

to

electron

been

against

reported

order

and

is

McLachlan

plotted

shifts

series

density

Carter,

are

In shift

the

series

shifts

measurements

shifts

chemical

this

shifts

also

from

n-electron

The striking

benzenes,

the

resonances

resonance of

both

6345.

Cotton

G. Fraenkel, R.E. Carter, Sot. 82, 5846 (1960).

3 H. Spiesecke

local

Cl3

mainly Hl chemical

shifts

by Fraenkel,

H1 shifts. nuclei,

and

between

members

1961)

aromatic

H1 chemical

In magnitude

of

* Issued

in

C 13 chemical

The corresponding

the

that

the

three

Council,

29 June

derived C 13

density

of which

Waughl,

corresponding

1 J.R.

the

first

shifts

2

suggest

n-electron

form

evidence,

measured

shown.

the

of

Research

revised

13 resonance C

the

p.

in

relationship

of

1 shows

to

each

IN AZULENE

Canada

quantitative

in

Cotton

1961;

amount

benzenes,

investigate density

2 June

a considerable

Printed

W.G. Schneider National

Ottawa, (Received

Ltd.

AND H1 NUCLEAR RESONANCE SHIFTS*

H. Spiesecke Division

Press

and

J.S.

Waugh,

A. McLachlan

W.G. Schneider,

Nature.Lond. and J.H.

J.Chem.Phvs.

L68

18Q, Richards,

In

press

978 (1957). J.Amer.Chem. (1%).

The determination

No.14 nuclei

respond in a parallel

of a-electron

469

densities

way to the local n-electron

density on the

carbon atom.

+50

+ IO

53

t2

s

x !!?

0 +-I

IO

-

c E -B

0

t E v)

I0

-30 -1

< 2 3 3 t

-2

0.9

0.9

1.1.

1.0

I.2

FIG. 1. The slope of the averaged lines factors,

SC13 = 160,~ and Bhl = 10.6~.

deviation *

in Fig. 1 yields

of the individual

the proportionality

In both cases,

points from the linear

however, the

correlation

line exceeds

A value of 160 p.p.m. per electron has also been deduced from Cl’ shifts by Lauterbur (private communication) by an indirect pror

470

The determination

the experimental

error

deviations

slightly

in the {different

resonance ponding

is not immediately

cyclic

nor is there coupling

ring

of the carbon shifts.

However, olefins

hybridization

and the consequent

1) reveals

The latter

in bond

to contribute

to the

no discernable

of the correstrend

in the magnitudes

with ring

size,

of the C-H spin-

have been shown to reflect

changes

in bond

compounds. 4,5

in other

Table C13 Chemical

Relative

varies

changes

of the C13 shifts

examination

explanation

The bond angle

may be expected

pattern

N0.U

A satisfactory

apparent.

systems

orbitals

(Table

any consistent

constants.

densities

of the B-measurements.

of these

hybridization

of a-electron

1

Shifts

of CH Groups in Olefinic

and Aromatic

Compounds -r-

SC13 Compound

JCH (C/S)

(p.p.m. relative to benzene)

Cyclooctatetraene

-4.1

155

Cycloheptatriene

-2.9,1.3,7.6

156,154,161

1,3_Cyclohexadien$

2.4 -4.2

163

Cyclopentadien

&

-9.5

158

42.5

145

-27.6

171

Rutadien& Cyclooctatetraene Tropylium

dianion

ion

Cyclopentadienyl

170

157

25.7

anion

L

a Two broad doublets p No resolved E Terminal Subject employed

shift

CH2 (1,4

to these

to evaluate

4 J.N.

Shoolery,

5 N. Muller

unresolved between

position

positions),

limitations the

local

2 and 3

tll.5

p.p.m.

the calibration n-electron

d.Chem.Phvs.

and D.E. Pritchard,

&,

plots

densities 47

of Fig.

of other

aromatic

(1959).

J.Chem.Phvs.

2,

1 may be

768 (1959).

systems.

-8.9 4.6 -8.9

498

597

6

Q13

JCH 183 165 158 161 158

7.5 -7.7 4.2 -8.6

(c/s)

-5.9

Corr *a

-0.170

iD.280

-0.945

-0.067

-0.583

Meas.

-0.03

+0.48

-0.34

+0.58

-0.23

c0rr.a

oH1

0.997

1.045

0.968

1.055

0.978

1112 (1956).

d R.D. Brown and M.L. Heffernan,Aust.J.Chem.2,

E A. Julg, J.Chim.Phvs.I;i,377 (1955).

b R. Pariser,J.Chem.Phvs.g,

38 (1960).

P

0.938 1.042

0.879 1.049 0.948 1.013

0.967 1.041 0.962

1.034

0.908

1.049

1.096

1.061

0.997

’

0.998

0.%9

1.011

0.954

1.059

0.988

Heffernan

Brow&

calculated Jul$

0.979

Parises

0.977

derived from 6$3

6Hl

P

a Corrected for shift due to ring current of neighbourring.

9.10

8.8

-6.7

Meas.

3,1

2

Posit- L ion

(Chemicalshifts in p.p.m. relative to benzene)

Cl3 and Hl Chemical Shifts and n-ElectronDensities of Azulene

Table 2

The determination

4772 Table

2

shows the results

hydrocarbon,

authors

and H’ shifts,

ring

values, 7

approximation. to be about derived

occurs

ently

resolved

in the azulene

molecule,

shown in Table

2,

being

relaxation

shift

carried

out on 2-3

cation,

which was measured

in the carbonyl

Cl3 resonance

shifts

The H’ resonance cyclohexane,

6

7

molar

solutions

for

these

The

roughly

was not

estimated

deviation independ-

no satisfactory

positions,

particular

dipole

the values

The largest

So far

of

a simple

between

which may be

nuclei.

in tetrahydrofuran,

as tropylium

fluoroborate

hydrolysis.

This

point

systems

were

Dimethylcarbonate

in a saturated

solution

in a 5 mole $ solution

signal

serving

of

benzene

to be converted

as an internal in cyclohexane

relative

tropylium

in aqueous

were measured

were measured

ring

except

reference.

shifts

shifts

the aromatic

as an external

the solvent

all

of

group was used

a 5 mole % solution

permitted

signals.

the 9,lO

measurements

some -IBF to prevent 4

containing

ment of

times

is

13

investigated.

The Cl3 resonance

$3

values

the Cl3 signal

by other

C

circulation

by employing

final

reported

The relative

current

correspondence

for

values

and must be corrected.

which

have been obtained

the non-alternant density

from the H1 and Cl3 resonances.

and was overlapped

due to unfavorable

by ring

of the

for

NO.L$

columns.

were obtained

accuracy

at the 6-position,

signals

manner for

three

15%. There is a very close

(-&)

is

last

influenced

The overall

densities

the calculated

are

independently

resonance

in this

are shown in the

as measured,

the neighbor corrected

obtained

For comparison

azulene.6

by several

of n-electron

solution enriched

in

The azulene of

azuIene

in CS 2’

of azulene

in

reference. in a similar

Measuremanner

to benzene.

The Cl3 resonance shifts of azulene have recently been reported by bauterbur [J.Amer.Chem.Soc. a, 1838 (1961)] and correlated with the theoretically calculated charge densities in azulene. J.A. Pople, W.G. Schneider Maanetic Resonance p. 254.

and H.J. Bernstein, Hiah Resolution McGraw-Hill, New York (1959).

Nuclear