Application of Computational Methods to the Chemistry of Lignin

Chapter 19

Application of Computational Methods to the Chemistry of Lignin Thomas Elder

Downloaded by UNIV OF SOUTHERN CALIFORNIA on April 17, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch019

School of Forestry, Auburn University, Auburn, AL 36849

Computational and theoretical techniques have been used to describe a wide range of compound classes, but have been only sparingly utilized in studies on the properties and reactions of lignin. A brief summary of the capabilities and limitations of molecular mechanics and molecular orbital calculations is presented, along with a survey of specific applications to lignin that have been reported in the literature. A n e x a m i n a t i o n of the other articles i n t h i s text serves as a n excellent i l l u s t r a t i o n of the diverse a n a l y t i c a l m e t h o d s t h a t have been successfully a p p l i e d to lignocellulosic m a t e r i a l s . T h e p r a c t i t i o n e r s of w o o d c h e m i s t r y have r a p i d l y a s s i m i l a t e d a n d a d a p t e d m o d e r n i n s t r u m e n t a l c h e m i s t r y to their specific p r o b l e m s . I n contrast, the techniques of c o m p u t a t i o n a l c h e m i s t r y have not been w i d e l y used i n such a n e n v i r o n m e n t . T h e c u r r e n t p a p e r w i l l a t t e m p t t o describe the c a p a b i l i t i e s , o p p o r t u n i t i e s , a n d l i m i t a t i o n s of s u c h a n a p p r o a c h , a n d discuss the results t h a t have been r e p o r t e d for l i g n i n related c o m p o u n d s . T h e t e r m " c o m p u t a t i o n a l c h e m i s t r y " can refer i n its broadest sense to a w i d e range of m e t h o d s t h a t have been developed t o give insight i n t o the f u n d a m e n t a l b e h a v i o r of c h e m i c a l species. S u c h m e t h o d s i n c l u d e , b u t are not necessarily l i m i t e d to, those related to q u a n t u m mechanics (1), m o l e c u l a r mechanics (or force-field c a l c u l a t i o n s ) (2), p e r t u r b a t i o n theory (3), g r a p h theory (4), or s t a t i s t i c a l t h e r m o d y n a m i c s (5). F o r the purposes of t h i s chapter, c o m m e n t s w i l l be r e s t r i c t e d to force-field a n d q u a n t u m based c a l c u l a t i o n s , since these are the techniques t h a t have been used i n w o r k o n l i g n i n . F u r t h e r m o r e , these methods have been reviewed i n a very readable b o o k b y C l a r k (6). A b r i e f p e r u s a l of the p r e v i o u s l y cited references w i l l reveal the h i g h l y i n v o l v e d n a t u r e of the m a t h e m a t i c a l f o r m a l i s m t h a t has led to these t e c h niques. T h i s p r o b l e m is offset, to some extent, b y the ready a v a i l a b i l i t y of 0097-6156/89/0397-0262$06.00/0 © 1989 American Chemical Society

Glasser and Sarkanen; Lignin ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


19.

ELDER

Application of Computational Methods

263

well-developed software, a n d i m p r o v e m e n t s i n the speed, capacity, a n d costs associated w i t h c o m p u t e r h a r d w a r e . These advances enable users to c o n centrate o n the i n t e r p r e t a t i o n of results, r a t h e r t h a n the intricacies o f the m a t h e m a t i c s . A w e a l t h of p r o g r a m s are available, for e n v i r o n m e n t s r a n g i n g f r o m p e r s o n a l c o m p u t e r s to s u p e r c o m p u t e r s , at very reasonable cost f r o m the Q u a n t u m C h e m i s t r y P r o g r a m E x c h a n g e at I n d i a n a U n i v e r s i t y . I n spite of the m i n i m a l a p p l i c a t i o n s of c o m p u t a t i o n a l c h e m i s t r y to the c h e m i s t r y of w o o d , the techniques have become h i g h l y developed a n d s o p h i s t i c a t e d i n t h e i r a b i l i t y t o calculate c h e m i c a l p r o p e r t i e s for a w i d e v a r i e t y o f c o m p o u n d classes. M e t h o d s based o n q u a n t u m m e c h a n i c s , c o m m o n l y referred to as m o l e c u l a r o r b i t a l c a l c u l a t i o n s , have been the t o p i c of n u m e r o u s b o o k s , reviews, a n d research papers ( 7 , 8 , 9 , 1 0 ) . These t e c h niques are concerned w i t h the d e s c r i p t i o n of electronic m o t i o n , a n d the s o l u t i o n of the Schrodinger e q u a t i o n to determine the energy of m o l e c u l a r systems. Since the exact s o l u t i o n of the Schrodinger e q u a t i o n is o n l y p o s s i ble for t w o - p a r t i c l e systems, a p p r o x i m a t i o n s must be i n v o k e d for even the simplest organic molecules. Molecular Orbital Calculations. T h e most s o p h i s t i c a t e d a n d t h e o r e t i c a l l y rigorous of the m o l e c u l a r o r b i t a l methods are ab initio c a l c u l a t i o n s . T h e s e are p e r f o r m e d w i t h a p a r t i c u l a r m a t h e m a t i c a l f u n c t i o n d e s c r i b i n g the shape of the a t o m i c o r b i t a l s w h i c h combine to p r o d u c e m o l e c u l a r o r b i t a l s . T h e s e f u n c t i o n s , or basis sets, m a y be chosen based o n a convenient m a t h e m a t i c a l f o r m , or their a b i l i t y to reproduce c h e m i c a l properties. C o m m o n l y used b a sis sets are a c o m p r o m i s e between these two extremes, b u t s t r i c t ab initio c a l c u l a t i o n s use o n l y these m a t h e m a t i c a l functions to describe electronic m o t i o n . Representative of ab initio methods is the series of G A U S S I A N p r o g r a m s f r o m C a r n e g i e - M e l l o n U n i v e r s i t y (11). In general, these c a l c u l a t i o n s are c o m p u t a t i o n a l l y quite intensive, a n d require a large a m o u n t of c o m p u t e r t i m e even for r e l a t i v e l y s m a l l molecules. In order to p e r f o r m c a l c u l a t i o n s on larger molecules i n a reasonable a m o u n t of t i m e , a p p r o x i m a t i o n s are m a d e , w h i c h m a y i n v o l v e the neglect of c e r t a i n terms, or the i n c l u s i o n of e x p e r i m e n t a l l y d e t e r m i n e d p a r a m e ters. T h e best k n o w n a n d simplest e x a m p l e of t h i s level of a p p r o x i m a t i o n are H i i c k e l M o l e c u l a r O r b i t a l ( H M O ) c a l c u l a t i o n s , w h i c h treat o n l y p i electrons, i n conjugated h y d r o c a r b o n s , w i t h neglect of overlap (1). W h i l e o b v i o u s l y l i m i t e d i n use, H M O m e t h o d s are s t i l l used i n c e r t a i n research applications. A v a r i e t y of m o r e advanced, all-electron methods of t h i s t y p e are a v a i l able, a n d are generally referred to as s e m i - e m p i r i c a l c a l c u l a t i o n s . T h e a c r o n y m s used to n a m e the i n d i v i d u a l m e t h o d s are descriptive of the m a n ner i n w h i c h a t o m i c overlap c a l c u l a t i o n s are p e r f o r m e d . A m o n g the m o r e w i d e l y used s e m i - e m p i r i c a l m e t h o d s are those of complete neglect of differential overlap ( C N D O / 2 ) (12), modified i n t e r m e d i a t e neglect of differe n t i a l overlap ( M I N D O / 3 ) (13), a n d m o d i f i e d neglect of d i a t o m i c overlap ( M N D O ) (14). T h e r e l a t i v e m e r i t s of a n u m b e r of m o l e c u l a r o r b i t a l m e t h o d s have been discussed i n the l i t e r a t u r e ( 1 5 , 1 6 , 6 ) . T h e m e t h o d s c i t e d differ f u n d a m e n -



264

LIGNIN: PROPERTIES AND MATERIALS

t a l l y i n t h a t the C N D O / 2 m e t h o d was developed to m i m i c the results of ab initio c a l c u l a t i o n s , w h i l e the l a t t e r t w o approaches are concerned w i t h the r e p r o d u c t i o n of e x p e r i m e n t a l results. It has been s a i d t h a t the a i m of D e w a r ' s g r o u p was to develop a n " M O spectrometer" t h a t c a n p r o v i d e results w i t h e x p e r i m e n t a l accuracy i n a reasonable t i m e (6). T h e results f r o m m o l e c u l a r o r b i t a l c a l c u l a t i o n s c a n be d i v i d e d i n t o two general categories, d e a l i n g w i t h energy a n d electronic p o p u l a t i o n s . T h e w e a l t h of i n f o r m a t i o n t h a t c a n be o b t a i n e d is i l l u s t r a t e d i n F i g u r e 1 (17). S u c h d a t a are useful i n the i n t e r p r e t a t i o n of e x p e r i m e n t a l results, a n d the p r e d i c t i o n of the course of novel reactions. Molecular Mechanics Calculations. I n sharp contrast t o the m o l e c u l a r orb i t a l c a l c u l a t i o n s t h a t are related to q u a n t u m mechanics, a n d a t t e m p t to describe the m o t i o n of electrons i n a f r a m e w o r k of fixed n u c l e i , m o l e c u l a r m e c h a n i c s , or force field, c a l c u l a t i o n s c o m p l e t e l y ignore the presence of electrons. T h e s e techniques, w h i c h have been reviewed b y O s a w a a n d M u s s o (18), B u r k e r t a n d A l l i n g e r (2), a n d A l l i n g e r (19), treat m o l e c u l a r systems classically, representing the a t o m s as masses a n d the b o n d s w h i c h connect t h e m as springs. I n the simplest f o r m , springs t h a t obey H o o k e ' s L a w are used, w h i l e i n m o r e c o m p l i c a t e d systems, s u c h as those i n v o l v i n g h y d r o g e n - b o n d e d i n t e r a c t i o n s , M o r s e p o t e n t i a l s or other f u n c t i o n s m a y be used ( 1 9 , 2 ) . M o l e c u l a r mechanics c a l c u l a t i o n s represent the energy of a c o m p o u n d as the s u m of the energy of s t r e t c h i n g , b e n d i n g , v i b r a t i o n , t o r s i o n , n o n - b o n d e d i n t e r a c t i o n s , a n d terms t h a t couple these m o t i o n s . I n i t i a l l y , m o l e c u l a r mechanics m e t h o d s were developed for the i n t e r p r e t a t i o n of v i b r a t i o n a l s p e c t r a , b u t m o r e recently they have been a p p l i e d to a n u m b e r of other p h e n o m e n a a n d properties (19). A n u m b e r of force-field m e t h o d s have been described (20-24), a n d w h i l e c o n c e p t u a l l y s i m i l a r , the p o t e n t i a l f u n c t i o n s a n d p a r a m e t e r i z a t i o n s t h a t are used m a y be q u i t e different (2). M o l e c u l a r mechanics c a l c u l a t i o n s have been c a r r i e d out o n a w i d e range of c h e m i c a l c o m p o u n d s i n c l u d i n g h y d r o c a r b o n s , h e t e r o a t o m i c molecules, steroids, c a r b o h y d r a t e s a n d proteins. F u r t h e r m o r e , a v a r i e t y of i n f o r m a t i o n has been o b t a i n e d such as heats of f o r m a t i o n , r o t a t i o n a l b a r r i e r s , a n d rates of r e a c t i o n ( 1 8 , 2 , 1 9 ) . T h e m a j o r advantage of t h i s m e t h o d over other c o m p u t a t i o n a l methods is t h a t i t is reasonably fast to p e r f o r m i n c o m p a r ison to f o r m a l m o l e c u l a r o r b i t a l c a l c u l a t i o n s .

Applications of Computational Methods to Lignin W i t h t h i s i n t r o d u c t i o n to the methods of c o m p u t a t i o n a l chemistry, the a t t e n t i o n of t h i s p a p e r w i l l now t u r n t o specific a p p l i c a t i o n s related to the c h e m i s t r y of l i g n i n . A s p r o m i s e d at the b e g i n n i n g of t h i s p a p e r , the discussion w i l l address not o n l y the capabilities a n d o p p o r t u n i t i e s t h a t m a y accrue f r o m t h i s t y p e of research, b u t w i l l also consider the l i m i t a t i o n s of the techniques. O n e of the basic a s s u m p t i o n s is t h a t a l l c a l c u l a t i o n s are done o n isol a t e d gas-phase molecules. T h i s is o b v i o u s l y a large s h o r t c o m i n g for l i g n i n c h e m i s t r y for a n u m b e r of reasons. T h e reactions of i m p o r t a n c e i n l i g n i n



19.

ELDER


265

Resonance energy

Energy of the molecule

Bond order

Thermodynamic stabiity

Electric charge ι—* Transition energies

Electron spectra Ionization potential

Highest occupied MO

MO LCAO

Length and energy of the bond: force constants: vibra tion frequencies

Bond order Free Coefficients of AO in MO

valence Electric charges

w

Reactivity, degree of conjugation Reactivity, cfpole moments

Transition probabilities

Spectral and optical properties

Figure 1: Electronic characters obtained by the molecular orbital method their applications.

(Reprinted

with

permission

from

ref.

17. Copyright

Academic Press.)


and 1977

266



w o r k , specifically those related to p u l p i n g a n d b l e a c h i n g , take place i n aque ous systems, a n d l i g n i n is a large heterogeneous p o l y m e r . T h e s o l v a t i o n questions m a y be overcome to some extent b y a d d i t i o n a l c a l c u l a t i o n s . F o r e x a m p l e , the m o l e c u l a r mechanics m e t h o d described b y W e i n e r a n d K o l l m a n (24) has the a b i l i t y to a p p l y a given dielectric constant to the molecule, a n d corrections to gas-phase c a l c u l a t i o n s have been r e p o r t e d (25,26). T h e i n d e t e r m i n a t e a n d p o l y m e r i c n a t u r e of l i g n i n c a n , of course, be addressed b y the u t i l i z a t i o n of j u d i c i o u s l y chosen m o d e l c o m p o u n d s . T h i s is u s u a l l y the strategy i n basic e x p e r i m e n t a l studies o n l i g n i n , a n d the same logic s h o u l d a p p l y to c o m p u t a t i o n a l methods. I n a d d i t i o n , the lack of basic t h e r m o d y n a m i c d a t a for c o m p o u n d s of interest is a l i m i t a t i o n , since i n most s i t u a t i o n s there are no e x p e r i m e n t a l values to w h i c h c o m p u t e d results m a y be c o m p a r e d . T h e v a l i d a t i o n proce dure t o w h i c h the m e t h o d s are subjected, however, includes a large range of c o m p o u n d s for w h i c h e x p e r i m e n t a l d a t a is d o c u m e n t e d . W h i l e t h i s is not direct evidence t h a t for specific c o m p o u n d s the results w i l l be repre sentative of e x p e r i m e n t a l results, i t is one of the a s s u m p t i o n s t h a t has been m a d e i n the work o n l i g n i n . T h e s e difficulties n o t w i t h s t a n d i n g , the methods of c o m p u t a t i o n a l c h e m i s t r y represent a u n i q u e a p p r o a c h to the questions of w o o d science, a n d r a t h e r t h a n a s u m m a r y d i s m i s s a l , s h o u l d be e x a m i n e d to determine their a p p l i c a b i l i t y . I n spite of such difficulties, t h e o r e t i c a l c a l c u l a t i o n s have been successfully used i n w o r k related to m a t e r i a l s science ( 2 7 , 2 8 ) , a n d i n n u m e r o u s b i o c h e m i c a l a p p l i c a t i o n s (29). T h e a p p l i c a t i o n of c o m p u t a t i o n a l methods to l i g n i n c h e m i s t r y are be i n g researched m a i n l y b y i n d i v i d u a l s a n d groups i n the U n i t e d States, a n d Soviet U n i o n , C z e c h o s l o v a k i a , a n d F i n l a n d . O n e of the earliest papers o n t h i s t o p i c was concerned w i t h the use of H i i c k e l M o l e c u l a r O r b i t a l m e t h o d s i n the s t u d y of l i g n i n r e a c t i v i t y (30). It was r e p o r t e d t h a t the c a l c u l a t e d l o c a l i z a t i o n energies were related to the r e a c t i o n rates of phenols, a n d t h a t c a l c u l a t i o n s o n free r a d i c a l s t r u c t u r e s c o u l d be of i m p o r t a n c e i n the e l u c i d a t i o n of factors t h a t influence the p o l y m e r i z a t i o n of l i g n i n precursors. F u r t h e r m o r e , G l a s s e r a n d co-workers (31-36) have used s i m u l a t i o n t e c h niques to m o d e l the p o l y m e r i z a t i o n a n d reactions of l i g n i n . In a s t u d y d i r e c t l y related to the c o u p l i n g of phenols to f o r m the l i g n i n p o l y m e r , M a r t e n s s o n a n d K a r l s s o n (37) used P a r i s e r - P a r r - P o p l e c a l c u l a tions to e x a m i n e the π-electron s p i n densities of a v a r i e t y of p h e n o x y r a d icals. T h e s p i n density, representative of u n p a i r e d electron character, was f o u n d to be consistently highest at the phenolic o x y g e n i n each of the r a d i c a l s t r u c t u r e s t h a t were considered. T h i s has been i n t e r p r e t e d as t h e o r e t i c a l evidence t h a t is i n accordance w i t h e x p e r i m e n t a l d a t a i n d i c a t i n g t h a t the /?-0-4 linkage is the p r e d o m i n a n t m o d e of c o m b i n a t i o n i n softwood l i g n i n . T h e a d d i t i o n of m e t h o x y , aroxy, a n d a r y l s u b s t i t u e n t s o r t h o to the p h e n o l i c oxygen was f o u n d to effectively d i l u t e the s p i n densities t h r o u g h o u t the r a d i c a l s t r u c t u r e s r e p o r t e d . In p a r t i c u l a r , the s p i n density of the p o s i t i o n p a r a to the phenolic oxygen seems to be reduced at the expense of the car b o n to w h i c h the s u b s t i t u e n t is a t t a c h e d . T h e r e l a t i v e l y h i g h s p i n density


19.

ELDER


267


at t h i s p o s i t i o n as a result of s u b s t i t u t i o n does n o t , however, represent a site o f elevated r e a c t i v i t y , because o f steric h i n d r a n c e , or t h e r m o d y n a m i c c o n s t r a i n t s (38). R e l a t e d research has been r e p o r t e d b y E l d e r a n d W o r l e y (39), i n w h i c h M N D O was used t o e x a m i n e the s t r u c t u r e of c o n i f e r y l a l c o h o l , a n d i t s c o r r e s p o n d i n g phenolate a n i o n a n d free r a d i c a l . T h i s m e t h o d represents a n i m p r o v e m e n t over the P P P m e t h o d , i n t h a t M N D O is a n a l l - e l e c t r o n t e c h n i q u e , a n d performs geometry o p t i m i z a t i o n s . It was f o u n d t h a t the c a l c u l a t e d s p i n densities a n d charge values for the reactive sites d i d not correlate q u a n t i t a t i v e l y w i t h observed b o n d frequency, b u t i t was observed t h a t p o s i t i o n s w i t h p a r t i a l negative charge a n d p o s i t i v e s p i n densities are the p o s i t i o n s t h r o u g h w h i c h the p o l y m e r i z a t i o n has been f o u n d t o o c c u r . T h e most extensive a n d s y s t e m a t i c s t u d y of the c h e m i s t r y of l i g n i n w i t h t h e o r e t i c a l m e t h o d s has been p e r f o r m e d b y R e m k o a n d co-workers. T h e i r w o r k has i n v o l v e d the n a t u r e of i n t r a m o l e c u l a r (40-43) a n d i n t e r m o l e c u l a r (44-48) h y d r o g e n b o n d i n g of l i g n i n m o d e l c o m p o u n d s , s p e c t r a l t r a n s i t i o n s (49-52), a n d c o n f o r m a t i o n a l a n a l y s i s (53). T h e m e t h o d s used have i n c l u d e d C N D O / 2 a n d P C I L O ( P e r t u r b a t i v e C o n f i g u r a t i o n I n t e r a c t i o n u s i n g L o c a l i z e d O r b i t a l s ) (54). C o n f o r m a t i o n a l studies of d i m e r i c l i g n i n m o d e l c o m p o u n d s have been r e p o r t e d b y R e m k o a n d S e k e r k a (53), a n d G r a v i t i s a n d E r i n s (55) f r o m the Soviet U n i o n . G r a v i t i s a n d E r i n s (55) used the p a i r w i s e a t o m - a t o m p o t e n t i a l f u n c t i o n ( A A P F ) m e t h o d , a force-field t e c h n i q u e , to d e t e r m i n e the c o n f o r m a t i o n of /J-0-4 l i n k e d d i m e r s . It was f o u n d t h a t the a r o m a t i c rings, i n the most stable a r r a n g e m e n t , e x h i b i t a folded s t r u c t u r e , w i t h the rings i n p a r a l l e l planes. I n a s i m i l a r , b u t s o m e w h a t m o r e extensive s t u d y of d i m e r i c l i g n i n m o d e l c o m p o u n d s , R e m k o a n d S e k e r k a (53), u s i n g P C I L O , f o u n d t h a t the lowest energy of the /?-0-4 d i m e r corresponded to t w o c o n f o r m a t i o n s , one of w h i c h was a p l a n a r s t r u c t u r e , w h i l e the other h a d a r o t a t i o n o f 120° between the a r o m a t i c r i n g s . A c c o r d i n g t o R e m k o a n d S e k e r k a (53), t h i s d i s c r e p a n c y is due to the u t i l i z a t i o n of different c o m p u t a t i o n a l m e t h o d s . F u r t h e r m o r e , a l t h o u g h the d i m e r s i n b o t h papers were β-0-4 l i n k e d , they were not i d e n t i c a l , a n d were not s u b s t i t u t e d w i t h h y d r o x y l or m e t h o x y l groups. In c o n j u n c t i o n w i t h a s t u d y o n the r e a c t i v i t y of d i m e r i c q u i n o n e m e thides, E l d e r et a l . (56) e x a m i n e d the p h y s i c a l a n d electronic s t r u c t u r e of g u a i a c y l g l y c e r o l - / ? - c o n i f e r y l ether, w h i c h is s u b s t i t u t e d i n a m a n n e r repre sentative o f the l i g n i n p o l y m e r . C a l c u l a t i o n s were p e r f o r m e d u s i n g A M B E R ( A s s i s t e d M o d e l B u i l d i n g w i t h E n e r g y Refinement) (24), w h i c h is a forcefield m e t h o d , a n d the energetic m i n i m u m was d e t e r m i n e d t o be a folded s t r u c t u r e s i m i l a r to t h a t r e p o r t e d by G r a v i t i s a n d E r i n s (55). S i n c e the properties o f a n y m a t e r i a l are r e l a t e d t o i t s s t r u c t u r e , the c o n f o r m a t i o n a l aspect of these studies cannot be neglected. T h e e m p h a s i s of the l a t t e r paper (56) was, however, p r i m a r i l y concerned w i t h the elec t r o n i c s t r u c t u r e of the d i m e r i c m o d e l c o m p o u n d , a n d how t h i s i n f o r m a t i o n m i g h t be c o m p a r e d to e x p e r i m e n t a l facts. U p o n c o m p l e t i o n o f the c a l c u l a t i o n s , i t b e g a n to appear t h a t the results were seriously flawed. It was


268



f o u n d , s u r p r i s i n g l y , t h a t the a l p h a c a r b o n of the q u i n o n e m e t h i d e has a slight negative charge. Since the p r i n c i p a l r e a c t i o n of k r a f t p u l p i n g is p r o posed to involve n u c l e o p h i l i c a t t a c k of t h i s p o s i t i o n by either s u l f h y d r y l or h y d r o x i d e ions, how can these electronic d a t a be reconciled? F u r t h e r e x a m i n a t i o n of the results i n d i c a t e d t h a t b y i n v o c a t i o n of P e a r s o n ' s H a r d - S o f t A c i d - B a s e ( H S A B ) t h e o r y (57), the results are c o n sistent w i t h e x p e r i m e n t a l o b s e r v a t i o n . A c c o r d i n g t o P e a r s o n ' s theory, w h i c h has been generalized t o i n c l u d e nucleophiles (bases) a n d electrophiles (acids), i n t e r a c t i o n s between h a r d reactants are p r o p o s e d t o be dependent o n c o u l o m b i c a t t r a c t i o n . T h e c o m b i n a t i o n of soft reactants, however, is t h o u g h t to be due t o overlap of the lowest u n o c c u p i e d m o l e c u l a r o r b i t a l ( L U M O ) of the electrophile a n d the highest o c c u p i e d m o l e c u l a r o r b i t a l ( H O M O ) of the nucleophile, the so-called frontier m o l e c u l a r o r b i t a l s . It was f o u n d t h a t , c o m p a r e d to a l l other positions i n the quinone m e t h i d e , the a l p h a c a r b o n h a d the greatest L U M O electron density. It appears, therefore, t h a t the frontier m o l e c u l a r o r b i t a l i n t e r a c t i o n s are o v e r r i d i n g the u n f a v o r a b l e c o u l o m b i c c o n d i t i o n s . T h i s i n t e r p r e t a t i o n also s u p p o r t s the preferential r e a c t i o n of the s u l f h y d r y l i o n over the h y d r o x i d e i o n i n k r a f t p u l p i n g . I n c o m p a r i s o n t o the h y d r o x i d e i o n , the s u l f h y d r y l is r e l a t i v e l y soft, a n d i n P e a r s o n ' s theory, soft reactants w i l l b o n d p r e f e r e n t i a l l y t o soft r e a c t a n t s , w h i l e h a r d acids w i l l favorably combine w i t h h a r d bases. Since the a l p h a p o s i t i o n is the softest i n the entire molecule, as evidenced b y the L U M O density, the softer s u l f h y d r y l i o n w o u l d be m o r e l i k e l y to a t t a c k t h i s p o s i t i o n t h a n the h y d r o x i d e . T h e influence of frontier m o l e c u l a r o r b i t a l s has also been d e m o n s t r a t e d i n work o n the c h l o r i n a t i o n of l i g n i n m o d e l c o m p o u n d s (58). I n t h i s w o r k , electronic parameters were c o m p a r e d to the difference i n energy between g r o u n d state molecules a n d c h l o r i n a t e d i n t e r m e d i a t e s . T h e results i n d i cated t h a t for c o n i f e r y l a l c o h o l , the p o s i t i o n e x h i b i t i n g the greatest negative charge is the m e t h o x y l oxygen, w h i l e the p o s i t i o n w i t h the greatest electron density i n highest o c c u p i e d m o l e c u l a r o r b i t a l area is p a r a to the p h e n o l i c h y d r o x y l . T h e H O M O is the frontier m o l e c u l a r o r b i t a l of interest i n t h i s r e a c t i o n , since the c o n i f e r y l a l c o h o l is the n u c l e o p h i l e a n d the c h l o r o n i u m i o n is the e l e c t r o p h i l e . F a v o r a b l e r e a c t i o n indices n o t w i t h s t a n d i n g , these p o s i t i o n s have the two highest energy barriers to c h l o r i n a t i o n . S i m i l a r l y , the ^ - c a r b o n i n the a l i p h a t i c s i d e c h a i n has the lowest energy b a r r i e r , b u t does not have cor r e s p o n d i n g l y great negative charge or H O M O electron density. It c a n be seen t h a t steric considerations m a y be s t r o n g l y i n f l u e n c i n g the r e a c t i v i t y o f these sites, w i t h the c a r b o n b e i n g l a r g e l y u n h i n d e r e d , w h i l e the others w i t h greater r e a c t i o n barriers are s t r o n g l y h i n d e r e d . If these three positions are neglected, l e a v i n g a r o m a t i c p o s i t i o n s a n d the α - c a r b o n of the s i d e c h a i n , i t is f o u n d t h a t the size o f the negative charge does not reflect the order of the energy differences. I n c o n t r a s t , the H O M O electron density at the various p o s i t i o n s increases i n the same sequence as the energy b a r r i e r t o w a r d c h l o r i n a t i o n . It m a y be c o n c l u d e d , therefore, t h a t the c h l o r i n a t i o n r e a c t i o n is m o r e sensitive t o w a r d o r b i t a l c o n t r o l , a n d steric a r g u m e n t s , t h a n s i m p l e c o u l o m b i c a t t r a c t i o n .



19.

ELDER


269

T h e u t i l i z a t i o n o f Pearson's hard-soft acid-base theory t o i n t e r p r e t the reactions o f l i g n i n has also been described i n w o r k f r o m the Soviet U n i o n b y Z a r u b i n a n d K i r y s u n (59). B e y o n d t h i s specifically related paper, researchers i n the Soviet U n i o n have been quite active i n the a p p l i c a t i o n o f n u m e r i c a l m e t h o d s t o l i g n i n - r e l a t e d problems ( 6 0 , 6 1 ) . I n a n a t t e m p t t o relate c a l c u l a t e d results t o e x p e r i m e n t a l findings f o r m o n o m e r i c , l i g n i n m o d e l c o m p o u n d s , p r e l i m i n a r y w o r k has c o m p a r e d t h e o r e t i c a l l y d e t e r m i n e d electron densities a n d c h e m i c a l shifts r e p o r t e d f r o m c a r b o n - 1 3 nuclear m a g n e t i c resonance spectroscopy (62). A l t h o u g h c h e m i c a l shifts are a f u n c t i o n o f numerous factors, o f w h i c h electron density is o n l y one, b o t h theoretical a n d e m p i r i c a l r e l a t i o n s h i p s o f t h i s n a t u r e have been explored for a v a r i e t y o f c o m p o u n d classes, a n d are reviewed b y E b r a heem a n d W e b b (63), M a r t i n et a l . (64), N e l s o n a n d W i l l i a m s (65), a n d F a r n u m (66). Regression analyses i n d i c a t e d t h a t a m o d e l o f C - 1 3 N M R shift v s . electron density, f o r a l l c o m p o u n d s a n d carbons, resulted i n a p o o r fit o f the d a t a w i t h a significance level o f 0.0879 a n d a n R - s q u a r e d o f 0.0347. I n c l u s i o n of a factor defining the p o s i t i o n of each carbon y i e l d e d a significant r e l a t i o n s h i p , b u t s t i l l gave low correlation coefficient o f 0.2508. S o r t i n g the d a t a b y each c a r b o n indicates significant linear r e l a t i o n s h i p s for carbons 4 a n d 6 w i t h i n t h e a r o m a t i c r i n g , a n d t h e a a n d β carbons i n t h e side c h a i n . These results m a y p r o v i d e a more general p i c t u r e for the r e a c t i v i t y of l i g n i n . F r o m t h i s brief i n t r o d u c t i o n , i t is obvious t h a t the e x e c u t i o n o f the c a l c u l a t i o n s a n d the i n t e r p r e t a t i o n o f the results are s t i l l i n their i n i t i a l stages. It is the contention o f the a u t h o r t h a t the insight into l i g n i n c h e m i s t r y t h a t m a y be o b t a i n e d b y these methods deserves careful c o n s i d e r a t i o n . Literature Cited 1. Lowe, John P. Quantum Chemistry; Academic Press, Inc.: New York, 1978. 2. Burkert, Ulrich; Allinger, N . L . Molecular Mechanics; A C S Monograph 177; American Chemical Society: Washington, D C , 1982. 3. Dewar, M . J. S.; Dougherty, R. C . The Theory of Organic Chemistry; Plenum Press: New York, 1975. 4. Trinajstic, N . Chemical Graph Theory; C R C Press: Boca Raton, F L , 1983. 5. McQuarrie, Donald A . Statistical Thermodynamics; Harper and Row, Inc.: New York, 1973. 6. Clark, T . A Handbook of Computational Chemistry. A Practical Guide to Chemical Structure and Energy Calculations; Wiley-Interscience: New York, 1985. 7. Dewar, M . J . S. The Molecular Orbital Theory of Organic Chemistry; McGraw-Hill: New York, 1969. 8. Flurry, Robert L . , Jr. Molecular Orbital Theories in Bonding of Or ganic Molecules; Marcel Dekker, Inc.: New York, 1968.



270


9. Murrell, J . N.; Harget, A . Semi-Empirical, Self-Consistent-Field Mole cular Orbital Theory of Molecules; Wiley-Interscience: New York, 1972. 10. Pople, J . Α.; Beveridge, D. L. 1970. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1977. 11. Binkley, J . S.; Whiteside, R. Α.; Hariharan, P. C.; Seeger, R.; DeFrees, D. J.; Schlegel, H. B.; Frisch, M. J.; Pople, J . Α.; Kahn, L. R. Gaussian 82 Release A. Carnegie-Mellon Univ., Pittsburgh, PA, 1982. 12. Pople, J. Α.; Segal, G. A . J. Chem. Phys. 1966, 4 4 , 3289. 13. Bingham, R. C.; Dewar, M. J . S.; Lo, D. H. J. Am. Chem. Soc. 1975, 9 7 , 1285. 14. Dewar, M . J . S.; Thiel, W. J. Am. Chem. Soc. 1977, 9 9 , 4899. 15. Halgren, Thomas Α.; Kleier, D. Α.; Hall, John H., Jr.; Brown, Leo D.; Lipscomb, W. N. J. Am. Chem. Soc. 1978, 1 0 0 , 6595. 16. Dewar, M . J . S.; Ford, G . P. J. Am. Chem. Soc. 1979, 1 0 1 , 5558. 17. Volkenstein, M . V . Molecular Biophysics; Academic Press: New York, 1977. 18. Osawa, Eiji; Musso, H. Angewante Chemie-International Edition in English 1983, 2 2 , 1. 19. Allinger, N. L. Adv. Phys. Org. Chem. 1976, 13, 1. 20. Lifson, S.; Warshel, A . J. Chem. Phys. 1968, 4 9 , 5116. 21. Engler, E . M.; Andose, J. M., Scheleyer, P. von R. J. Am. Chem. Soc. 1973, 9 5 , 8005. 22. Fitzwater, S.; Bartel, L. S. J. Am. Chem. Soc. 1976, 9 8 , 5107. 23. Allinger, N. L. J. Am. Chem. Soc. 1977, 9 9 , 8127. 24. Weiner, Paul K.; Kollman, Peter A . J. Comput. Chem. 1981, 2, 287. 25. Tvaroska, Igor; Kosar, Τ. Am. Chem. Soc. 1980, 1 0 2 , 6929. 26. Tvaroska, I. Biopolymers 1982, 2 1 , 1887. 27. Hayns, M . R. Int. J. Quantum Chem. 1982, 2 1 , 217. 28. Calais, Jean-Louis. Int. J. Quantum Chem. 1982, 2 1 , 231. 29. Lowdin, Per-Olov. Proceedings of the International Symposium on Quantum Biology and Quantum Pharmacology; John Wiley & Sons: New York, 1987. 30. Lindberg, J. Johann; Henriksson, A. Fin. Kemist. Medd. 1970, 7 9 , 30. 31. Glasser, Wolfgang G.; Glasser, H. Macromolecules 1974, 7, 17. 32. Glasser, Wolfgang G.; Glasser, H. Holzforschung 1974, 2 8 , 5. 33. Glasser, Wolfgang G.; Glasser, H. Cell. Chem. Tech. 1976, 1 0 , 23. 34. Glasser, Wolfgang G.; Glasser, H. Cell. Chem. Tech. 1976, 10, 39. 35. Glasser, Wolfgang G.; Glasser, H.; Nimz, H. Macromolecules 1976, 9, 866. 36. Glasser, Wolfgang G.; Glasser, H.; Morohoshi, N. Macromolecules 1981, 1 4 , 253. 37. Martensson, O.; Karlsson, G. Arkiv Kemi 1968, 3 1 , 5. 38. Glasser, Wolfgang G. In Pulp and Paper, Third Edition; Casey, James P., Ed.; John Wiley & Sons: New York, 1980. 39. Elder, T . J . ; Worley, S. D. Wood Sci. Tech. 1984, 1 8 , 307. 40. Remko, Milan; Polcin, J . Chem. Zvesti. 1976, 3 0 , 170. 41. Remko, Milan; Polcin, J. Z. Phys. Chem. (Leipzig) 1977, 2 5 8 , 219.



19.

42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66.

ELDER


271

Remko, Milan; Polcin, J . Z. Phys. Chem. (Neue Folge) 1980, 120, 1. Remko, Milan. Adv. Mol. Relaxation and Int. Proc. 1979, 14, 315. Remko, Milan; Polcin, J . Z. Phys. Chem.(NeueFolge) 1977, 106, 249. Remko, Milan; Polcin, J. Z. Phys. Chem. (Neue Folge) 1981, 125, 175. Remko, Milan; Polcin, J . Z. Phys. Chem. (Neue Folge) 1981, 126, 195. Remko, Milan. Adv. Mol. Relaxation and Int. Proc. 1979, 14, 37. Remko, Milan. Z. Phys. Chem. (Neue Folge) 1983, 134, 129. Remko, Milan; Polcin, J . Chem. Zvesti 1977, 31, 171. Remko, Milan; Polcin, J . Monatsh. Chem. 1977, 108, 1313. Remko, Milan; Polcin, J . Z. Naturforsch 1977, 32a, 59. Remko, Milan; Polcin, J . Collect. Czech. Chem. Commun. 1980, 45, 201. Remko, Milan; Sekerka, I. Z. Phys. Chem. (Neue Folge) 1983, 134, 135. Diner, S.; Malrieu, J. P.; Jordan, F.; Gilbert, M . Theor. Chim. Acta. 1969, 15, 100. Gravitis, J . ; Erins, P. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1983, 37, 421. Elder, T . J.; McKee, M . L.; Worley, S. D. Holzforschung 1988, 42, 233. Fleming, Ian. Frontier Orbitals and Organic Chemical Reactions; John Wiley & Sons: London, 1976. Elder, T . J.; Worley, S. D. Holzforschung 1985, 39, 173. Zarubin, M . Ya.; Kirysun, M . F. Fourth International Symposium on Wood and Pulping Chemistry, 1987, p. 407. Jakobsons, J . , Gravitis, J . Khim. Drev. 1985, 1985, 110 (Chemical Abstracts 103 : 87243). Gravitis, J . ; Kokorevics, Α.; Ozols-Kalnins, V. Khim. Drev. 1986, 1986, 107 (Chemical Abstracts 104 : 131736). Kringstad, K. P.; Mörck, R. Holzforschung 1983, 37, 237. Ebraheem, Κ. A. K.; Webb, G . A. Prog. NMR Sped. 1977, 11, 149. Martin, G . J.; Martin, M . L.; Odiot, S. Org. Mag. Res. 1975, 7, 2. Nelson, G . L.; Williams, E . A. Prog. Phys. Org. Chem. 1976, 12, 229. Farnum, D. G . Adv. Phys. Org. Chem. 1975, 11, 123.

RECEIVED May 29,1989


Application of Computational Methods to the Chemistry of Lignin

Recommend Documents