ESR and NMR spectra of the 2,6-dimethoxyphenyl-t -butylnitroxide radical. Experimental determination of coupling parameters by independent methods

Tetrahedron Letters Uo.57, pp.5003-5006, 1970.

ESR

AND

NMR

RADICAL.

SPECTRA

OF

THE

EXPERIMENTAL

Pergamon Press.

2,6-DIMETHOXYPHENYL-4BUTYLNITROXIDE

DETERMINATION

OF COUPLING

INDEPENDENT

H.J. Department

of

Jakobsen

Organic

PARAMETERS

BY

METHODS

and

Kurt

Torssell*

Chemistry,

8000

Printed in Great Britain.

Aarhus

University

C.

of Aarhus

Denmark

(Received in u( 2 November 1970; accepted for publication 15 November 1970)

In

view

hindered

of

aryl

recent

in

situ

describede4

in

these

In

servation

of

resolution

distribution

in

tally

unhindered

trast

to

of

tually ger

the more

or

One

simplified

the

ring

series:

less

has is

that

gauss,

nitroxide

about

the

of

Ia,

closely

feature &-butyl

is

gauss)

arising

gauss, from

a line-width

twice

give

the

limited

by

the

the

The

spin

in

sharp

hindrance.

expect

a basic

spectrum

the to

ring,

s The 3 x use

when

reasons

14 line of

group

for

these

effects

deuterated

a 3 x 3 line 5003

nitro

spectrum

spectrum

of

Ia

and

is

con-

this

1, Ib)

de-

rotated are

&-nitrosobutanes (Figure

steri-

density

the

prosmaller

N 2:1:2. This also stands a : a :a 0 m E in nitro-aromatic anions, where the spin the

the

meta-pro-

the

in

ob-

&-butyl

only.

to

con-

in

uniformly

from

rise

is much

effect

different

spectra

analysis

a-proton

gen-

other

ESR

radicals

i.e..

of

rotating,

is usually

broadening

completely

the

the

freely

the

those

in

was

previously

protons

ortho-disubstituted 0.7

that

as

complicating group

(No.1

of

resembled

a A common

0.3-0.4

thus

Furthermore, the

but

is

the

spectrum

sterically

2,6_dimethoxyphenyl-

2,6_dimethoxyiodobenzene

reported,

In

for

on

that

steric

should

from

coupling

generally the

The

doubtful.

splitting

constant

investigations

be

the

instrument.

by

obtained.

about

ESR

reported'

radicals

where

situation

conjugation

obscure.

of

whereas

and

t-butylnitroxidee.

hyperfine

the

and

that

hindered

a coupling

gauss)

to

NMR

xv4 the data

cavity

from

aryl

this

coupling,

creases

ESR

radicals,

of

have

(4.1

the

splitting

siderably.

ton

seemed

sterically

a hyperfine

tons

Ia,

differed

2,6-disubstituted of

from

&-butylnitroxides,

t-butylnitroxide, erated

results

out

still was

ac-

as

scaven-

from

which

No.57

y.m

the

meta-proton

of 4.1-0.2 radical

coupling

gauss

constant

emanating

was

from

the

determined.

A

para-proton

secondary

could

concentrations.

also

doublet be

observed

OCH,

tact 100

experimental

do

not

shifts.' MHz

lation

on

Ia

data with

was

a Varian

frequency

TABLE

Protons

agree

of

are

the

therefore

collected

earlier

synthesized6

spectrometer,

13 KHz

(Figure

Shiftsa the

Line position from TMS (Hz), Ia

in Table

reported

HA-100

1. Contact

Intensity

1. However,

values and

lH

operating

2).

NMR

spectra

the

of

radicals

Line

=

coupling

from

HR-mode

NMR

with

are

Ia

Calc.h_fs. (gauss) Ia

concon-

characterized

for

hfs. from ESR(gauss), Ia and Ib

-567

+1267

210

-0.17

0.1-0.2

m-H

2

+6ol4

-5314

825

+0.71

0.69

t-Butyl

9

-2216

+2326

630

-0.31

0.33

OCH,

6

+670

-292

220

+0.04

very

Hz

and

broad

signals

extraordinary

determination

of

with large

signs

and

chemical

shift

a line

width

shifts,

the

magnitudes

data

of Ref.3

at half

peak

direction

and

of

the

hyperfine

for

the

height

of

magnitude coupling

at

a modu-

1

aIn Hz at 100 MHz. b Calculated using the hydroxylamine.

d,

recorded

Constants obtained 13.3 gauss

width

(Ha),

R = 4-Bu,

spectrum

in

and Hyperfine Splitting Radicals Ia and Ib, aN Shift (AH) fro$ hydroxylamine (Ha)

NMR

R = L-BU

Ib

the

calculated

the

Ia

E-H

by

low

CHsO hv RNO-

stants

at

.

CHsO

The

splitting

corresponding

several

hundred

of which constants,

allow

eqn.

(11.’ = -3.73

aH I

AH&

x

lo+

Hz

(T = 295oK)

a

(1)

5005

No.57

AH. 1 nucleus

is i

the

with

shift

(positive

reference

to

in

its

the

upfield

position

in

direction)

a

related

in

ppm

diamagnetic

for

a

magnetic

compound.

a

FIG.l.

ESR Spectrum denoted the nitroxide.

FIG.2.

In tons

was

observed assignments

the

NMR

detected by

spectrum at

Kreilick.s of

NMR

Kreilick

of

+6014

Spectrum

la Hz

the

of

shown

The of

in

Ia

presence

(relative

Furthermore, were

of Ib. Spectrum

from to

be

CDCl,

of

to

a

internal

the in

peaks marked "a" di-&-butyl d,

(ca.

broad

40$

signal

TMS),

ESR

spectra

error.

The

w/w).

a of

values

from

the

meta

signal

which

Ia

Ib

and

resul.ting

pro-

was

the

not

NMR

from

the

.5006

NMR

No.57

and

ESR

perimental

spectra

the

signment I.e.,

interest like

carbon

leaks

of

NMR

the

= +0.04

a&H,

methoxy NMR

of

downfield

ring

the

present

investigation

agree

within

the

limit

of

ex-

error.

A point shifted

of

protons

spectrum,

of

is an

that

the

peak

ortho-methyl

out

on

the

absorption gauss

+670 Hz

supported

= +0.21

our

TMS)

agrees

of the

that

is

too

spin

its

the

is

density

sign.

methoxy

shifted

almost

protons

unpaired

to

finding

-nitroxide

This

gauss.

the

conservation

(from

by

ortho-methoxy

A.e.

with

bis-(E-methoxyphenyl)

asCH

the

group,

oxygen

at

is

for

The

as-

protons,

peak

for

downfield

well

on

with

the in

the

its

value

9 of

0.21

gauss

determined

by

Acknowledgement.

We

thank

nical

assistance

in

recording

grant

from

Danish

ESR

Dr.

Natural

measurements.s

K.

University

Schaumburg,

the

Science

NMR

spectra.

Research

This

of Copenhagen,

work

was

supported

for by

techa

Council.

REFERENCES

A., 1. Calder, Mol.Phys.

Forrester, l&3, 481

A.R., (1970).

2. Pedersen,

J.A.

and

3. Kreilick,

R.N.

J.Chem.Phys.

4. Torssell,

K.

5. Geske,

D.H.

6. Hoffman, 7. McConnel, 8.

Fischer,

A.,

Torssell,

Tetrahedron and

Ragle,

Feldman,

H.M. P.H.H.

and and

Emsley,

K.

To

9, 26,

J.L. A.M.,

Chesnut, Neugebaur,

J.W.,

be

1922

2759

Luckhurst,

Storey,

R.A.

(1966).

(1970).

Gelblum,

D.B.

and

published.

J.Am.Chem.Soc. and

G.R.,

2, E.

J.Chem.Phys.

F.A.

3532

(1961).

J.Am.Chem.Soc. 28,

Z.Naturforsch.

107 z,

86,

646

(1958). 1036

(1966).

(1964).