Nuclear Magnetic Resonance Spectrometry in Archaeology

12 Nuclear Magnetic Resonance Spectrometry

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in Archaeology C U R T W. B E C K , C O N S T A N C E A. F E L L O W S , and E D I T H M A C K E N N A N

Vassar College, Poughkeepsie, Ν. Y. 12601 Proton magnetic resonance (PMR) spectrometry supple ments optical spectroscopy and gas chromatography in the identification of organic archaeological materials. PMR establishes the average chain length and unsaturation of fatty acids. An oil sample of the sixth-fourth century B.C. is shown to consist largely of oleic acid. A solid fat of the third century A.D. contained principally myristic and pal mitic acids. The absence of measurable glyceryl signals shows both samples were at least 95% hydrolyzed. PMR spectra also indicate that the pyrolysate of Baltic amber contains significantly less p-cymene than that of modern pine resins, confirming previous evidence that abietane structures are only a minor component of the fossil resin.

T n l i t t l e m o r e t h a n a d e c a d e , n u c l e a r m a g n e t i c resonance ( N M R ) specA

t r o m e t r y has b e c o m e a n i n d i s p e n s a b l e t o o l of o r g a n i c c h e m i s t r y , u s e d

as r o u t i n e l y as i n f r a r e d spectroscopy w h i c h i t s u p p l e m e n t s r a t h e r t h a n replaces. 1 7

0,

1 9

A t o m i c n u c l e i w i t h o d d mass n u m b e r s s u c h as Ή ,

F , and

3 1

n u m b e r s s u c h as H , B , a n d 2

signals.

1 3

C,

1 5

N,

P a n d a l l a t o m i c n u c l e i w i t h e v e n mass b u t o d d a t o m i c 1 0

1 4

N have magnetic moments a n d give N M R

O f these, c a r b o n a n d h y d r o g e n are f o u n d i n p r a c t i c a l l y a l l

organic compounds,

but while

1 3

C m a g n e t i c resonance

(CMR)

spec

t r o m e t r y r e q u i r e s v e r y expensive e q u i p m e n t , t h e i n s t r u m e n t a t i o n of p r o t o n m a g n e t i c resonance ( P M R ) s p e c t r o m e t r y is n o w w i t h i n the

financial

r e a c h of most l a b o r a t o r i e s . I n b r i e f , a m a g n e t i c a t o m i c n u c l e u s i n a m a g n e t i c field m a y b e e i t h e r l i n e d u p w i t h o r o p p o s e d to the e x t e r n a l field, a n d the t r a n s i t i o n f r o m one state t o the other corresponds to a n a m o u n t of e n e r g y w h i c h c a n b e provided

by

electromagnetic

radiation

i n the r a d i o f r e q u e n c y

226 Beck; Archaeological Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

range.

12.

BECK

E T

AL.

N MR

227

Spectrometry

N M R i n s t r u m e n t s use either constant m a g n e t i c fields a n d v a r i a b l e r a d i o frequencies or constant frequencies a n d v a r i a b l e m a g n e t i c fields, b u t the abscissa of (cps)

N M R spectra is a l w a y s m e a s u r e d

i n u n i t s of

frequency

i n r e l a t i o n to a n a d d e d reference c o m p o u n d s u c h as t e t r a m e t h y l -

silane ( T M S ) a n d d i v i d e d b y the r a d i o f r e q u e n c y of the i n s t r u m e n t to y i e l d dimensionless n u m b e r s o n either the δ-scale, w h i c h sets the r e f e r ence s i g n a l e q u a l to zero, or o n the τ-scale, w h i c h sets it e q u a l to 10;


thus δ - j - τ =

10. T h e spectra i n this r e p o r t use the δ scale a n d s h o w t h e

reference s i g n a l of T M S at δ =

0.

T h e p r i m a r y a n d s e c o n d a r y l i t e r a t u r e of N M R s p e c t r o m e t r y is f a r too l a r g e to p e r m i t e v e n a r e c a p i t u l a t i o n here ( I , 2 ) .

W e w i l l attempt

no m o r e t h a n a v e r y b r i e f s u m m a r y of the o b s e r v a b l e p a r a m e t e r s a n d t h e i r significance. Chemical Shift. T h e p o s i t i o n of the resonance s i g n a l of a p r o t o n ( or of a g r o u p of i d e n t i c a l protons ) is a n i n d i c a t i o n of the m o l e c u l a r e n v i r o n m e n t of that p r o t o n a n d p e r m i t s , e.g., the d i s t i n c t i o n of a l i p h a t i c , a l l y l i c , olefinic, a n d a r o m a t i c h y d r o g e n s . Integral. T h e area u n d e r a n N M R s i g n a l ( o r a g r o u p of s i g n a l s ) is a m e a s u r e of the n u m b e r of h y d r o g e n atoms w h i c h are r e s p o n s i b l e for t h a t signal. Spin-Spin Splitting. T h e s p l i t t i n g of a s i g n a l i n t o t w o , three, f o u r , or m o r e peaks w h i c h s h o w a b i n o m i a l d i s t r i b u t i o n p a t t e r n is a n i n d i c a t i o n of t h e n u m b e r of h y d r o g e n atoms o n n e i g h b o r i n g c a r b o n atoms w h i c h change the effective m a g n e t i c e n v i r o n m e n t of the p r o t o n u n d e r o b s e r v a t i o n b y s m a l l b u t p r e d i c t a b l e amounts. Coupling Constants.

T h e distance b e t w e e n the i n d i v i d u a l peaks of

a s p l i t N M R s i g n a l is a f u r t h e r i n d i c a t i o n of the m o l e c u l a r e n v i r o n m e n t of the o b s e r v e d protons.

F o r e x a m p l e , protons o n n e i g h b o r i n g s a t u r a t e d

c a r b o n atoms split e a c h others' signals w i t h c o u p l i n g constants of the o r d e r of 6 cps; protons o n n e i g h b o r i n g olefinic c a r b o n atoms o r d i n a r i l y h a v e c o u p l i n g constants w h i c h are s i g n i f i c a n t l y greater. S i n c e the n u m b e r of h y d r o g e n atoms i n a n average o r g a n i c m o l e c u l e is q u i t e l a r g e , most N M R spectra are f a i r l y c o m p l e x .

Since m a n y similar

protons w i l l g i v e signals w i t h n e a r l y the same c h e m i c a l shifts, r e s o l u t i o n is often i n c o m p l e t e .

T h e use of l a n t h a n i d e shift reagents is a recent

a d v a n c e w h i c h goes far t o w a r d s o l v i n g the latter p r o b l e m : B y c o o r d i n a t i n g a r a r e e a r t h c o m p l e x w i t h the substrate m o l e c u l e , protons are des h i e l d e d i n inverse p r o p o r t i o n to the t h i r d p o w e r of t h e i r d i s t a n c e f r o m the c o o r d i n a t i o n site, a n d thus the signals are s p r e a d o v e r a w i d e r r a n g e . T h u s N M R spectrometry can y i e l d information about the structure, a n d e v e n the c o n f o r m a t i o n , of p u r e c o m p o u n d s

of c o n s i d e r a b l e c o m p l e x i t y .

M i x t u r e s , of course, confront the spectroscopist w i t h s p e c i a l p r o b l e m s , b u t e v e n h e r e the N M R t e c h n i q u e c a n often d e a l w i t h the q u a n t i t a t i v e

Beck; Archaeological Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

228

ARCHAEOLOGICAL CHEMISTRY

analysis of, for e x a m p l e , m i x t u r e s of isomers m o r e effectively a n d effort lessly t h a n c a n i n f r a r e d spectroscopy. It is this aspect w h i c h m a k e s N M R spectrometry

p a r t i c u l a r l y a t t r a c t i v e to

archaeological

chemistry.

The

a r c h a e o l o g i c a l chemist is often c o n f r o n t e d w i t h v e r y s m a l l a m o u n t s of c o m p l e x m i x t u r e s . I f the m a t e r i a l is o r g a n i c , the first step is to e s t a b l i s h its g e n e r a l n a t u r e : is i t a fat or o i l , a p i t c h or tar, a r e s i n or g u m ? W h i l e s u c h b r o a d d i s t i n c t i o n s as w e l l as t h e final i d e n t i f i c a t i o n of the c o n s t i t u


ents c a n be m a d e b y v a r i o u s a n a l y t i c a l m e t h o d s , w e h a v e f o u n d N M R s p e c t r o m e t r y u n i q u e l y u s e f u l i n a n u m b e r of recent studies. Hydrolyzed

Vegetable

Oil

I n the N e w a r k M u s e u m there is a s m a l l glass flask w h i c h is d a t e d as s i x t h - f o u r t h c e n t u r y B . C . w i t h a g e n e r a l p r o v e n a n c e of S y r i a . f o u n d to c o n t a i n a d a r k b r o w n l i q u i d .

It w a s

T h e contents w e r e r e m o v e d

by

R o b e r t H . B r i l l of the C o r n i n g M u s e u m of G l a s s a n d sent to us for analysis. A n N M R s p e c t r u m of the substance d i s s o l v e d i n d e u t e r o c h l o r o f o r m ( F i g u r e 1) i m m e d i a t e l y s h o w e d it to be p r e d o m i n a n t l y oleic a c i d ( o r a s o l u b l e m e t a l o l e a t e ) , p r o b a b l y w i t h some a d m i x t u r e of s a t u r a t e d fatty acids. T h e signals c a n be assigned as f o l l o w s . T h e t r i p l e t at 0.8 δ arises f r o m three protons of a t e r m i n a l m e t h y l g r o u p a t t a c h e d to a m e t h y l e n e g r o u p . T h e large, b r o a d singlet at 1.1 δ represents 22 protons o n m e t h y l ene g r o u p s w h i c h are n o t d e s h i e l d e d b y n e i g h b o r i n g sp

carbon

2

atoms

a n d w h i c h are so s i m i l a r to one another that n o m e a s u r a b l e s p l i t t i n g occurs.

T h e i l l - r e s o l v e d peaks b e t w e e n

1.6 a n d 3.0 δ are c a u s e d

by

m e t h y l e n e groups w h i c h are d e s h i e l d e d b y n e i g h b o r i n g s p c a r b o n atoms. 2

T h e c a r b o x y l g r o u p accounts for one s u c h m e t h y l e n e g r o u p .

7

Figure 1.

6

5

4

3

2

Oil from Syrian glass bottle, sixth-fourth B.C.

Since the

0

century


12.

BECK

NMR

E T A L .

229

Spectrometry

i n t e g r a l i n this area i n d i c a t e s less t h a n six protons, there c a n b e n o m o r e t h a n t w o other d e s h i e l d e d m e t h y l e n e g r o u p s ; h e n c e there is a t m o s t o n e d o u b l e b o n d p e r m o l e c u l e . T h e t w o v i n y l i c protons of this d o u b l e b o n d p r o d u c e t h e s i g n a l at a b o u t 5.4 δ. T h e i n t e g r a l of this s i g n a l corresponds to o n l y 1.3 protons.

T h e deficiency

of d e s h i e l d e d m e t h y l e n e a n d of

v i n y l i c protons i n d i c a t e s a n a d m i x t u r e of s a t u r a t e d f a t t y acids. O l e i c a c i d is the p r i n c i p a l f a t t y a c i d of o l i v e o i l ( 8 3 % ) , w h i c h also


contains s m a l l e r amounts of t h e s a t u r a t e d p a l m i t i c ( 6 % ) (4%)

a n d stearic

a n d of t h e d o u b l y u n s a t u r a t e d l i n o l e i c a c i d ( 7 % ) .

TheN M R

s p e c t r u m of t h e a r c h a e o l o g i c a l s a m p l e w a s closely m a t c h e d b y spectra of c o m m e r c i a l o l e i c a c i d ( F i g u r e 2 ) as w e l l as b y m i x t u r e s o f t h e a c i d w i t h its s o d i u m a n d p o t a s s i u m salts ( F i g u r e 3 ) .

j

ι

8

7

ι 6

ι 5

ι

Figure 2.

J

1

8

7

Figure 3.

1 6

1 5

ι

4

2

1

1

L

0

Oleic acid

1 4

1

3

I 3

I 2

I

L 1

0

Oleic acid containing 21 % sodium oleate


230


I n spite of its c o n s i d e r a b l e age, one m i g h t suspect that t h e a r c h a e o l o g i c a l m a t e r i a l c o u l d h a v e r e t a i n e d some u n s a p o n i f i e d o l i v e o i l , a n d w e therefore a t t e m p t e d to e s t a b l i s h the l i m i t s of detection of the u n s a p o n i f i e d o i l i n the presence of free fatty acids or salts. T h e N M R s p e c t r u m ( F i g u r e 4 ) of the s i m p l e s t t r i g l y c e r i d e , t r i a c e t i n ( t h e ester of g l y c e r o l w i t h acetic a c i d ) shows the resonance of t h e f o u r protons of the m e t h y l e n e groups of g l y c e r o l as a m u l t i p l e t at a b o u t 4.3 δ ( t h e m u l t i p l i c i t y of t h e s i g n a l is


caused b y the magnetic non-equivalence methylene

group).

The

of the t w o protons

single t e r t i a r y p r o t o n

i n each

of the g l y c e r y l

group

appears essentially as a q u i n t e t at a b o u t 5.3 δ. T h e latter p o s i t i o n is the same as t h a t of t h e v i n y l protons i n o l e i c a c i d , a n d the t w o signals c a n n o t b e u s e d to d i s t i n g l i s h t h e g l y c e r y l ester a n d the free f a t t y a c i d o n o u r instrument.

T h e N M R s p e c t r u m of fresh o l i v e o i l ( F i g u r e 5 )

confirms

t h i s ; the v i n y l protons of the u n s a t u r a t e d f a t t y a c i d a n d the t e r t i a r y p r o -

Figure 4.

Figure 5.

Triacetin

Italian olive oil



12.

BECK

NMR

ET AL.

231

Spectrometry

A. 7

8

6

5

Figure 6. ton

4

3

2

0

Oleic acid containing 10% olive oil

of the g l y c e r o l m o i e t y c o i n c i d e , b u t the s i g n a l of f o u r

methylene

protons is c l e a r l y v i s i b l e . I n a s y n t h e t i c m i x t u r e of o l e i c a c i d c o n t a i n i n g 10%

olive oil (corresponding

to 0 . 5 %

free g l y c e r o l )

(Figure 6),

the

g l y c e r y l protons at 4.3 δ are b a r e l y v i s i b l e as a d i s t u r b a n c e i n the baseline at l o w a m p l i t u d e s , b u t after a m p l i f i c a t i o n t h e y are c l e a r l y i d e n t i f i a b l e i n spite of the large a m o u n t of noise w h i c h is the p r i c e of e l e c t r o n i c a m p l i fication.

However, 5%

o l i v e o i l ( c o r r e s p o n d i n g to 0 . 2 5 % free g l y c e r o l )

can no l o n g e r b e d e t e c t e d b y o u r V a r i a n T - 6 0 i n s t r u m e n t . T h e l i m i t s of detection

could

be

lowered

substantially b y

using a more

sensitive

spectrometer a n d a t i m e - a v e r a g i n g c o m p u t e r w h i c h stores m a n y r e p e a t e d s p e c t r a l runs a n d p r i n t s out t h e i r s u m ; the signals g r o w i n p r o p o r t i o n to the n u m b e r of scans s u p e r i m p o s e d w h i l e the noise increases o n l y i n p r o p o r t i o n to the square root of that n u m b e r , so t h a t 100 r e p e a t e d r u n s w i l l i m p r o v e the signal-to-noise r a t i o b y a factor of 10. T h e i d e n t i f i c a t i o n of the a n c i e n t o i l s a m p l e was c o n f i r m e d b y other a n a l y t i c a l t e c h n i q u e s , i n c l u d i n g i n f r a r e d spectroscopy, t i t r a t i o n , a n d gas chromatography.

A f u l l a c c o u n t w i l l be p u b l i s h e d e l s e w h e r e ; o u r p u r

pose h e r e is to s h o w h o w m u c h u s e f u l i n f o r m a t i o n c o u l d b e g a i n e d f r o m N M R s p e c t r o m e t r y alone. Hydrolyzed

Animal

Fat

A n o t h e r a p p l i c a t i o n concerns t h e s o l i d r e s i d u e i n a p i l g r i m flask of the t h i r d c e n t u r y A . D . f r o m t h e R h i n e l a n d , w h i c h was also r e f e r r e d to us b y R o b e r t H . B r i l l . A b o u t 7 0 % of the m a t e r i a l was s o l u b l e i n c a r b o n t e t r a c h l o r i d e , a n d its N M R s p e c t r u m ( F i g u r e 7 ) i m m e d i a t e l y i d e n t i f i e d it as a m i x t u r e of s a t u r a t e d fatty acids ( a n d t h e i r s o l u b l e salts) w i t h a n average c h a i n l e n g t h of 14 to 16 c a r b o n atoms, c o r r e s p o n d i n g to m y r i s t i c and

p a l m i t i c acids.

T h i s a v e r a g e m o l e c u l a r w e i g h t was o b t a i n e d


by


232


J

I

8

I

7

6

Figure 7.

I

I

5

4

I

3

I

I

2

L

1

0

Fat from pilgrim flask, third century A . D .

c o m p a r i n g t h e i n t e g r a l of the t e r m i n a l m e t h y l g r o u p ( w h i c h is k n o w n to c o n t a i n three p r o t o n s )

a n d the i n t e g r a l of the α-methylene

group

( w h i c h is k n o w n to c o n t a i n t w o p r o t o n s ) w i t h the i n t e g r a l of the l a r g e singlet p r o d u c e d b y essentially i d e n t i c a l n o r m a l m e t h y l e n e groups the s a t u r a t e d straight c h a i n .

of

T h i s m e t h o d c a n n o t b e h i g h l y accurate

because the signals u s e d are too close to p e r m i t p r e c i s e m e a s u r e m e n t of t h e i r i n t e g r a l s . F u r t h e r m o r e the a n s w e r o b t a i n e d is necessarily a n a v e r age c h a i n l e n g t h of a m i x t u r e ; t h e a c t u a l c o m p o s i t i o n m u s t b e e s t a b l i s h e d b y gas c h r o m a t o g r a p h y of the m e t h y l esters.

I n the present case the

latter t e c h n i q u e s h o w e d t h a t m y r i s t i c a c i d is i n d e e d the p r i n c i p a l f a t t y a c i d present. H o w e v e r , N M R s p e c t r o m e t r y c a n be u s e d v e r y effectively to d e t e r m i n e the c h a i n l e n g t h , a n d h e n c e i d e n t i f y the a c i d , of a s i n g l e p u r e c o m p o n e n t after c h r o m a t o g r a p h i c s e p a r a t i o n ; the m e t h y l ester g r o u p i n t r o d u c e s a n e w s i g n a l at a b o u t 3.7 δ w h i c h is w e l l s e p a r a t e d f r o m a l l other signals a n d c a n therefore b e i n t e g r a t e d p r e c i s e l y . c o n t a i n three protons.

It is k n o w n to

A c o m p a r i s o n of its i n t e g r a l w i t h the i n t e g r a l of

a l l t h e other signals c o m b i n e d is a n excellent m e a s u r e of c h a i n l e n g t h as s h o w n b y the e x a m p l e of m e t h y l p a l m i t a t e ( F i g u r e 8 ) w h i c h shows a n e r r o r of less t h a n one p r o t o n . The Structure

of

Amber

A n o t h e r p r o m i s i n g a p p l i c a t i o n of N M R s p e c t r o m e t r y to a r c h a e o l o g i c a l p r o b l e m s deals w i t h fossil resins. T h e s e t e r p e n o i d p o l y m e r s , the best k n o w n of w h i c h is a m b e r , h a v e p l a y e d a large r o l e i n t r a d e relations d u r i n g p r e h i s t o r i c times since at least t h e n e o l i t h i c e r a , a n d the deter m i n a t i o n of t h e i r g e o g r a p h i c a n d b o t a n i c a l o r i g i n c a n c o n t r i b u t e to o u r k n o w l e d g e of this e a r l y c o m m e r c e .


12.

NMR

BECK E T A L .

233

Spectrometry

Palaeobotanists h a v e assigned t h e source tree of t h e B a l t i c a m b e r o f n o r t h e r n E u r o p e to a n extinct species of p i n e , Firms succinifera,

on purely

m o r p h o l o g i c a l g r o u n d s b u t h a v e b e e n u n a b l e to relate this tree to a n y l i v i n g p i n e species

( 3 ) . T h i s assignment has l e d some investigators t o

t h e hypothesis that B a l t i c a m b e r m a y b e a d e r i v a t i v e o f t h e p r i n c i p a l r e s i n a c i d of most c o m m o n i n f r a r e d spectroscopic

m o d e r n p i n e s — a b i e t i c a c i d (4).

Chemical and

e v i d e n c e raises some serious difficulties w i t h this

v i e w ( 5 ) b u t w i t h o u t b e i n g a b l e to resolve t h e q u e s t i o n .

One crucial


p o i n t concerns t h e presence of i s o p r o p y l groups i n B a l t i c a m b e r .

Such

groups are present i n a b i e t i c a c i d a n d i n a l l t h e p o l y m e r i c d e r i v a t i v e s o f a b i e t i c a c i d w h i c h h a v e b e e n c o n s i d e r e d as constituents of f o s s i l a m b e r , b u t t h e y a r e absent i n t h e p r i n c i p a l r e s i n acids of m a n y other possible source trees, i n c l u d i n g n o t o n l y other conifers b u t also N o r t h A m e r i c a n a n d A s i a t i c pines w h i c h m i g h t w e l l h a v e

flourished

i n the m i l d climate

of t h e E u r o p e a n T e r t i a r y p e r i o d . U n f o r t u n a t e l y , fossil resins a r e l a r g e l y i n s o l u b l e b e c a u s e of t h e i r p o l y m e r i c character, a n d a d i s s o l v e d s a m p l e is a p r e r e q u i s i t e f o r a w e l l resolved N M R spectrum.

W e h a v e therefore

carried out comparative

N M R studies o n t h e s o l u b l e pyrolysates of a m b e r a n d of p i n e resins.

I

1

8

ι

1

7

6

ι 5

Figure 8.

ι 4

Methyl

ι 3

ι 2

ι 1

L J 0

palmitate

T h e p y r o l y s i s of p i n e r o s i n y i e l d s p i n e o i l w h i c h has l o n g

been

k n o w n ( 6 ) to c o n t a i n l a r g e amounts of p - c y m e n e ( p - i s o p r o p y l t o l u e n e , bp, 177°C).

W e have prepared pine o i l b y d r y distillation of commercial

r o s i n a n d f r a c t i o n a t e d i t at a t m o s p h e r i c pressure i n t o 10° cuts. T h e N M R s p e c t r u m of t h e f r a c t i o n b o i l i n g at 1 7 0 ° - 1 8 0 ° C ( F i g u r e 9 ) shows t h a t p - c y m e n e is i n d e e d t h e m a j o r constituent present.

T h e N M R spectrum

of p u r e p - c y m e n e has b e e n r e p o r t e d ( 7 ) , a n d its signals are p r o m i n e n t i n the s p e c t r u m of p i n e o i l . T h e d o u b l e t at 1.22 δ, w i t h a c o u p l i n g constant


234


of 7 cps, is c a u s e d b y the t w o i d e n t i c a l m e t h y l groups of the i s o p r o p y l structure, split b y the n e i g h b o r i n g t e r t i a r y p r o t o n .

T h i s latter p r o t o n

accounts f o r the m u l t i p l e t ( t h e o r e t i c a l l y a septet) at 2.85 δ. T h e s i n g l e t of the t o l u e n e - m e t h y l g r o u p appears at 2.30 δ i n p u r e p - c y m e n e .

The

s p e c t r u m of the p i n e o i l f r a c t i o n shows t w o singlets, at 2.28 a n d 2.29 δ, respectively.

E i t h e r m a y be the p - c y m e n e

m e t h y l g r o u p ; the other is

m o s t l i k e l y c a u s e d b y a n a d m i x t u r e of the isomer m - c y m e n e .

T h e pres


ence of other isomers is also e v i d e n t f r o m the m u l t i p l i c i t y of the a r o m a t i c r i n g protons. W h i l e p u r e p - c y m e n e gives a s h a r p singlet at 7.08 δ, p i n e o i l shows s p l i t t i n g of the b e n z e n o i d protons w h i c h a g a i n m a y be

ac

c o u n t e d for b y the presence of the m e t a isomer. B o t h isomers w o u l d b e e x p e c t e d f r o m the p y r o l y t i c cleavage of a n a b i e t i c a c i d m o l e c u l e . T h e same signals of p - c y m e n e

are also present, a l t h o u g h at l o w e r intensities, i n

the N M R spectra o f the p i n e o i l fractions b o i l i n g at 1 6 0 ° - 1 7 0 ° C , 190°C, and 190°-200°C.

180°-

T h u s N M R offers a s i m p l e means of e s t a b l i s h

i n g the presence of i s o p r o p y l groups i n resins of the a b i e t i c a c i d t y p e .

J 8

1 7

Figure 9.

1 6

1 5

ι 4

ι 3

ι

ι

2

Pine oil fraction, boiling point

L 1

0

170°-180°C

O i l of a m b e r o b t a i n e d b y d e s t r u c t i v e d i s t i l l a t i o n of B a l t i c y i e l d e d o n l y a v e r y s m a l l f r a c t i o n b o i l i n g i n t h e r a n g e of T h e N M R s p e c t r u m of this f r a c t i o n ( F i g u r e 10)

amber

160°-180°C.

lacks the p r o m i n e n t

i s o p r o p y l d o u b l e t . I n its a p p r o x i m a t e p l a c e there appears a c h a r a c t e r i s t i c s i g n a l of three e v e n l y s p a c e d peaks. It c a n n o t b e a p r o p e r t r i p l e t because the distance b e t w e e n the peaks is o n l y a b o u t 3.5 cps a n d because

the

intensity of the peaks is not i n the r e q u i r e d 1:2:1 r a t i o . I t seems t h e r e fore l i k e l y t h a t this t r i p l e s i g n a l is m e r e l y a n a c c i d e n t a l s u p e r i m p o s i t i o n of a d o u b l e t h a v i n g a n o r m a l c o u p l i n g constant of 7 cps w i t h another q u i t e u n r e l a t e d singlet. I f so, the d o u b l e t m a y s t i l l be the r e s u l t of a n i s o p r o p y l g r o u p , b u t the w e i g h t s of the fractions a n d t h e i r spectra s h o w



12.

NMR

BECK E T A L .

8

6

7

Figure 10.

235

Spectrometry

5

4

Oil of amber fraction,

Ο

2

3

boiling point

that a m b e r y i e l d s s i g n i f i c a n t l y less p - c y m e n e

160°-180°C

t h a n does r e s i n . C u r r e n t

w o r k o n the i s o l a t i o n o f i n d i v i d u a l a l k y l b e n z e n e s f r o m r e s i n p y r o l y s a t e s f o l l o w e d b y N M R analysis o f the p u r e c o m p o n e n t s structures f o r t h e constituents o f B a l t i c a m b e r . NMR

spectroscopy

has c o n f i r m e d

that there

may provide partial E v e n now, however,

a r e essential s t r u c t u r a l

differences b e t w e e n B a l t i c a m b e r a n d the a b i e t i c a c i d resins o f t h e c o m m o n species o f l i v i n g pines.

T h e s e f e w r a n d o m examples i l l u s t r a t e t h e

p o t e n t i a l c o n t r i b u t i o n s w h i c h N M R s p e c t r o m e t r y is a b l e t o m a k e to t h e archaeological chemistry of organic materials. Literature Cited 1. Roberts, J. D., "Nuclear Magnetic Resonance: Applications to Organic Chemistry," McGraw-Hill, New York, 1959. 2. Bible, R. H., "Interpretation of NMR Spectra; An Empirical Approach," Plenum Press, New York, 1965. 3. Schubert, K., Beiheft z. Geol. Jahrb. (1961) 45. 4. Rottländer, R. C. Α., Archaeometry (1970) 12, 35. 5. Beck, C. W., Naturwissenschaften (1972) 59, 294. 6. Kelbe, W., Ann. (1882) 210, 1. 7. Varian Associates, "High-Resolution NMR Spectra Catalog," 1962, spec trum 268. RECEIVED July 9, 1973.


Nuclear Magnetic Resonance Spectrometry in Archaeology

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