A C T A U N I V E R S I T A T I S L O D Z I E N S I S F OL IA C H I M I C A 10, 1993 Cez ar y M. K i n ar t* S T U D I E S O N I N T E R M O L E C U L A R I N T E R A C T I O N S IN P Y R I D I N E - W A T E R L I Q U I D M I X T U R E S B Y M E A N S OF LH - N M R S P E C T R A A N D E L E C T R I C P E R M I T T I V I T Y V A L U E S This p ap er c o n t i n u e s my p r e v i o u s s t u d i e s [l-4] on liquid b i n ar y mi x tu re s . The 1H- NMR s p e ct ra d i s c u s s e d h e r e i n were n e wly r e c o r d e d for nine b i n ar y m i x t u r e of p y r i d i n e (Py) wi th w at er (H 20) at 290 .1 5 K, w i t h i n the p o s s i b l y full range of c o m p o s i t i o n (i.e. from 2.42 to 66.79 m o l % of Py). S u b s e que ntly , b a s ed on these s pe c tr a l data, the r e l a t i v e d i f f e ren ce s in the c he m i c a l shift v al ues we re m e a s u r e d b et w e e n the c e n te r of the H 20 p r o t o n s sig nal and the c e n te r of m u l t i p l e t d e r i v e d from the five Py a r o m a t i c p r o t o n s (i.e. 8(Py - H 20)). U s i n g the same m e t h o d as d e s c r i b e d p r e v i o u s l y
[l-4] from these new s pe c t r a l data the values of the s pe c tr a l p a r a m et e r, A 6 (Py - H 20), were d e t e r mi n ed . I n d e p e n d e n t l y , the val ue s of the t e m p e r a t u r e c o e f f i c i e n t s of e l e c t r i c p e r m i t t i vities, £ , viz. a = (l/£) • [d£ /d(l/T)], w er e c a l c u l a t e d at 298.15 K w i t h i n the full range of c o m p o s i t i o n s of the s tu d i e d b i n ar y m ix ture s. The a na l y s i s of the r es u l t s thus o b t a i n e d as well as the a v a i l a b l e l i t e r a t u r e r e p o r t s w ou ld i nd i ca t e the f o r m a t i o n of so me r e l a t i v e l y most s t a b le " co m p l e x e s " ( d i h y d r a t e s ) , Py • 2 H 20, in the l iq uid m i x t u r e s u nd er i n v e s t i g a t i o n .
I have c h o s e n the said b i n a ry m i x t u r e s for the p r e se nt study, since m o l e c u l e s of w at er and p i r i d i n e due to the k n o w n s t r u c t u r e s of their m o l e c u l e s are apt to s t r on g int er - m o l e c u l a r i n t e r a c t i o n s m a i nl y by h y d r o g e n b oi ndin g.
E X P E R I M E N T A L
C h e m i c a l l y p ur e p y r i d i n e (Fluka) was d r y i e d and p u r i f i e d a c c o r d i n g to the k n o wn p r o c e d u r e [5] . Red isti lled water from a quartz
g la ss d i s t i l l i n g d e v i c e was used. All the b i n a ry s o l u t i o n s s t u died, Py - H 20, w er e p r e p a r e d by weight. The lH -N MR s p e ct ra (at 2 9 8 .1 5 K + 1 K) w er e r e c o r d e d on a Tesla s p e c t r o m e t e r of the type BS 487 C (80 MHz). The c he m i c a l shift val ue s for the p r o to n s i g n a l s of Py and H 20 in e ac h b in ary m i x t u r e were m e a s u r e d with an a cc u r a c y of +0.2 Hz, in r e s pe ct to an e xt e r n a l s t a n d ar d , HMDS ( h e x a m e t h y l d i s i l o x a n e ) .
R E S U L T S ANO D I S C U S S I O N
U s i n g the same m e t h o d as p r e v i o u s l y [1-4] new data of the s p e c t r a l p a r a m et e r, A 6 (Py - H 20), were d e t e r mi n ed . The l oc a t i o n of a m a x i m u m in the g r a p h p l o t t e d vs. the c o m p o s i t i o n of the m i x e d s o l v e n t p o i n t s at a p a r t i c u l a r c o m p o s i t i o n w h e r e the s t r o n g e s t i n t e r m o l e c u l a r i n t e r a c t i o n s b e t we en the c o m p o n e n t s are d i s p l ay e d. The par ent, 6 (Py - H 20) data as well as the r e s p e c t i v e A 5 ( P y - H 20) v al ues are s h o w n in Tab. 1 and are v i s u a l i z e d in Fig. 1 as a f u n c t i o n of the m i x t u r e c o m p o s i t io ns .
L at er on, u si ng the lit e ra t ur e, t e m p e r a t u r e - d e p e n d e n t , data of e l e c t r i c p e r m i t t i v i t i e s ( E ^ ) m e a s u r e d over the w h o l e ran ge of s o l v e n t c o m p o s i t i o n s for the Py - H 2 0 m i x t u r e s [5-7], I have c a l c u l a t e d the v a l ue s of the t e m p e r a t u r e c o e f f i c i e n t s of £ (at 298. 15 K), d e n o t e d of, viz. a = (1/E) • [ d £ / d ( 1 / T )]. The m a x i m u m of this c oe f f i c i e n t , in a g r e e m e n t with the r e s u l t s s u b m i t t e d by R a e t z s c h [8] and b a s e d on some t h e r m o d y n a m i c r e a s on i ng , p o i n t s at a "c o mp le x " wi th the s t r o n g e s t i n t e r m o l e c u l a r i n t e r a c tion b e t w e e n the c om p o n e n t s . C h a n g e s of oi as a f u n c t i o n of the m i x t u r e c o m p o s i t i o n s are v i s u a l i z e d in Fig. 2.
In the p r e v i o u s p a p er s [l-4] I ha ve s ho wn that the re is a f ai rl y good a g r e e m e n t in c o n c l u s i o n s d r a w n from both the plots of the s pe c t r a l A S p a r a m e t e r and of the ot p ro pert y. B e f o re c o n s i d e r i n g the s i g n i f i c a n c e of r e s ul ts o b t a i n e d in this work, it is n e c e s s a r y to ma ke a b ri ef r e v ie w of the l i t e r a t u r e o p i n i o n s p e r t a i n i n g to m u t u a l i n t e r a c t i o n s b e t w e e n m o l e c u l e s of p y r i d i n e and w at er in their liq ui d mi x tu re s . D u n s t o n , T h o l e and H u n t [9] had m e a s u r e d d e n s i t i e s for the Py - H 2 0 m i x t u r e s , at 298 K, and p l o t t e d a c ur ve s h o w i n g a n u m b er of c h a r a c t e r i s t i c f e a t u r e s w h i c h we re a t t r i b u t e d to the e x i s t e n c e of s e v e r a l
dif-% mol. Py
Fig. 1. C h a ng es in the fun ctio ns, S ( P y - f^O) = f (mol%) and A S ( P y - H 90) = f (mol%), for the liq ui d p y r i d i n e - w a t e r mi x tu re s ,
L at 290 .15 K
fe rent h y d r a t e s in a q u e o u s s o l u t io n s of p yr idin e. H a r t l e y , T h o m a s and A p p l e b e y [10] have r e p e a t e d the same d e n s i t y m e a s u r e m e n t s , and their d e n s i t y show no o th er c h a r a c t e r i s ti c f ea t ur e s but only one m a x i m u m at ca. 30 m o l % of Py. Th e re fo r e, the e x i s t e n c e of a sole d i h y d ra t e, Py • 2H 2O, is s u g g e s t e d by them. Also, the d e n si ty c u r v e o b t a i n e d by T r e w ard S p e n c e r [ll] s ho ws only a s i n gl e m ax imum , w hi ch c o r r a b o - r at es the r e s a l t s in [10]. The m e n t i o n e d abo ve a ut h o r s h av e also o bs e rv e d, by p l o t t i n g the d e n si ty c u r v e s at d i f f e r e n t t e m p e r a t u res, that the m a x i m a are s h i ft ed t o w ar ds lower p y r i d i n e c o n c e n t r a t i o n s w it h i n c r e a s i n g the tem p er a tu re . Thus, at 273 K the
T a b l e 1 R e l a t i v e c h e m i c a l shifts, 6 (Py - H o 0), and H N MR s pe c t r a l parameters, Afi(Py
-- H 20), m e a s u r e d at 2 9 8 . 15 K m ol % of P S ( P y - H 20) [Hz] AS( By - H 20) [Hz] 2.42 182.5 0.0 5.38 176.5 4.5 8.90 172.0 8.3 13.20 162.5 12.0 18.27 157.0 14.3 25.11 151.5 16.0 34.27 145.5 16.4 48.40 140. 5 10.5 66.79 138. 5 0.0
m a x i m u m a lm ost c o r r e s p o n d s to 30 mo l% of Py, and it is s h i ft ed at 298 K to ca. 20 m o l % of Py. A c le ar d i s t i n c t i o n of the c o m p o s i t i o n of a " c o m p le x " to w h i c h the r e s p e c t i v e m a x i m u m was due was then m ad e by p l o t t i n g the d e v i a t i o n s of the d e n s i t y val ues fr om the v a l ue s c a l c u l a t e d by the a dd i t i v e law of m i x t u r e s a g a i n s t the c o m p o s i t i o n , s in ce in this way the e f f e ct of the r e l a t iv e t e m p e r a t u r e c o e f f i c i e n t s of w at er and p y r i d i n e w o u l d be p o s s i b l y e l i m i n a t e d . The r e s u l t s o b t a i n e d by T r e w and S p e n c e r [ll] p oi nt at the e x i s t e n c e of a d i h y d r a t e Py • 2 H 20. A c c o r d i n g to them, that c o n c l u s i o n s is c o n f i r m e d by the c o r r e s p o n d i n g p r o p e r t y - c o m p o s i t i o n c u r v e s for r e f r a c t i v e i n d ic es [12], volume c o n t r a c t i o n s [13], vap ou r p r e s s u r e s [14] , and v i s c o s i t i e s [15], all of w hi ch are fou nd to d i s p l a y a m a x i m u m at the c o m p o s i t i o n equal to ca. 30 mo l% of Py. Very i n t e r e s t i n g d i s c u s s i o n c o n c e r n i n g this p r o b l e m was later on p r e s e n t e d by C i n i and T a d d e i [16]. They have a na l y s e d c h a n g e s in d e n s i t i e s and s u s c e p t i b i l i t i e s of the w a t e r - p y r i d i n e l i q ui d m i x tures over the full c o n c e n t r a t i o n range, at 299 K. They have i n t e r p r e t e d their r e s u l t s as an e v i d e n c e for the e x i s t e n c e of
"com-% mol. Py
Fig. 2. C ha n g e s in the t e m p e r a t u r e c o e f f i c i e n t s of d i e l e c t r i c p e r m i t t i v i t y d ra wn as a f un c ti o n of c o m p o s i t i o n for the l iq uid
p y r i d i n e - w a t e r m i x t u r e s at 298 .1 5 K
p l e x es " ( d i h y d r a t e s ) of the type Py • 2 H 20. They ha ve a ls o been very c r i t i c a l as to r e l i a b i l i t y of the r e s u l t s p r e v i o u s l y obtained by H a r t l e y [10] and T r e w [ l l ] . In turn, R i z k and S h i n o u d a [17] h av e d e n ie d the r e s u l t s o b t a i n e d by C i n i and T a d d e i [16] who p o s t u l a t e d a p o s s i b i l i t y of f o r m a t i o n of h y d r o g e n b on ds b e t w e e n m o l e c u l e s of p y r i d i n e in n ea t l i q u id Py. Rizk a n a l y z e d the K i r k w o o d p a r a m e t e r in pure p yr i di n e, w h e r e a s H o l m e s at al. [18] e x c l u d e d a p o s s i b i l i ty of the p r e s e n c e of s h o r t - r a n g e i n t e r m o l e c u l à r for ces in neat Py as well as its s e l f - a s s o c i a t i o n there. T h e r e fo r e, they have a s s um ed that the c o n c l u s i o n s d ra wn by C i n i and T a d- d e i [16] are not cor rect .
F u r t h e r on, the s c r u t i n y of p r o p e r t i e s of the w a t e r - p y r i d i n e m i x t u r e s c a r r i e d out by R i z k and S h i n o u d a [l7, 19] was b a s e d on the m e a s u r e m e n t s of d e n s i t i e s for the Py - H 2 0 l i q ui d s y s t e m w i t h i n the t e m p e r a t u r e ran ge 2 8 3 -3 33 K. F ro m those p a r e n t data they p l o t t e d the s p e c i f i c v ol ume curves, the molar v o l um e c u r v e s and d e v i a t i o n s of the m ol ar volumes from "id eali ty". Fr om the r e s u l t s they d e d u c e d the sole e x i s t e n c e of a sta ble d i h y d r a t e , i.e. Py • 2 H 20. As to c o n f i r m this c o n c l u s i o n they ha ve also a n a l y s e d d e v i a t i o n s of the e l e c t r i c p e r m i t t i v i t i e s £12 from a d d i t i v i t y and v a r i a t i o n s of the d i e l e c t r i c r e l a x a t i o n t imes (t) for v a r ie d w a t e r - p y r i d i n e m i x t u r e s w i t hi n the t e m p e r a ture r an ge 2 8 3 -3 33 K. A c c o r d i n g to them, the d i e l e c t r i c r e l a x a tion times r ea ch their m a x i m u m at 30 m ol % of Py; this m a x i m u m is c l e a r l y seen at 283 K, and it n e a r l y d i s s a p e a r s at 333 K. Similar ly, the £12 v al ues p l o t t e d as a f u n c t i o n of the c o m p o s i t i o n r e ach a deep m i n i m u m a ls o at ca. 3,0 m ol % of Py quite id e nt ic a l b e h a v i o u r is e x h i b i t e d by the v i s c o s i t y values, T21 2 , a s w e l l as by c a l c u l a t e d from t he m the r e s p e c t i v e r e l a x a t i o n times. R i z k
[17] us ed the m e a s u r e d val ue s of v i s c o s i t i e s and e l e c t r i c p e r m i t t i v i t i e s to c a l c u l a t e the a c t i v a t i o n e n e r g i e s for d i p o l e r e lax atio n, E £ , and the a c t i v a t i o n e n e r gy for v i s co us flow, E^. They o b t a i n e d the se v a l ue s from the f o l l o w i n g two e q u a t io n s:
T = f • exp [E £ /R • (T - Tq )]
xi = B • exp [E^/R • (T - Tq )]
Both Ej. and E^ r ea ch their m a x i ma at 33 m ol % of Py in the s t u d i e d Py - H 2 0 mi x tu re s . The m o d i f i e d A r r h e n i u s e q u a t i o n given b e l o w also has been a p p l i e d to c a l c u l a t e the val ue s of V f ("free v o l u m e s " ) for the s t u d i e d b i n a ry mi x tu re s , over the t e m p e r a t u r e ra nge from 283 to 333 K.
‘V , * V vi ■<T - v ■R/E,
where: V f - "free volume", V i - s pe c i f i c volume, V i = V12 = l/d^. V a l u es of (V^,) p l o t t e d as a f un c t i o n of c o m p o s i t i o n of the m i x e d s o l ve nt pass t h r ou gh a m i n i m u m at ca. 35 m ol % of Py, w h e r e a s v al ues of ( V d i s p l a y a m i n i m u m 32 m ol % of Py. These m i n i m u m a are s hi f t e d t o w ar ds lower c o n t e n t s of water wi th i n c r e a s i n g the tem p er a tu re . C o h e n and T u r m b u l l [20] have s u g ge st that "free vo l um es " are p r o p o r t i o n a l to the c ha n g e s in t em p era ture. A c c o r d i n g to R i z k [l7] all these o b s e r v a tions c o n f i r m the h y p o t h e s i s that some p a r t i c u l a r c o m p o s i t i o n w he re the s t r o n g e s t i n t e r m o l e c u l a r h y d r o g e n bo nd i n t e r a c t i o n s b e t w e e n p y r i d i n e and w at er are d i s p l a y e d c o r r e s p o n d s to ca. 33 morn of Py and that the s t a b le h y d r o g e n - b o n d e d "co mple x" ( di h ydr ate), Py • 2 H 20, is the only p o s s i b l e one c o m p l e x in the s t u d i e d m ix t ure s. A d o n et al. [21] m e a s u r e d v a r i a t i o n s of the e n t r o p i e s of m i x i n g over the w ho le r an ge of s o l ve nt c o m p o s i tions. D e v i a t i o n s of this f un c ti o n from a d d i t i v i t y show a m i n i m um at ca. 33 m o l % of Py. The same s u g g e s t i o n can be d r a w n from
IR m e a s u r e m e n t s in the s pe c t r a l reg ion b e l ow 3400 c m "1 [22-24] as well as from the e n e r g e t i c m e a s u r e m e n t s b as ed on a n a l y s i s of the 1H - NMR s p e ct ra [22, 23, 25, 26]. The e l e c t r o c h e m i c a l s t u di es c a r r i e d wi th the Py - H 20 m i x t u r e s by S i n g h , M a c L e o d and P a r k e r [27] seem to be also w or th m en t i o n i n g . They h av e r ep o r t e d v a l ue s of the l im i t i n g c o n d u c t a n c e for NaNO-j, AgNOj, CuNO-j and C u ( N 0 j )2 over the w h o l e r an ge of the c o m p o s i t i o n s of the m i x e d sol vent , Py ■ 2 H 20, at 298 K. The l im i t i n g c o n d u c t a n ces A e x h i b i t for all the s t u d i e d e l e c t r o l y t e s , a m i n i m u m at
’ o
ca. 33 mo l% of Py. It m ea ns that this m i n i m u m is o b s e r v e d for the same c o m p o s i t i o n r e g i on for w h i ch the p r e v i o u s l y m e n t i o n e d a u t h o r s have a s s u m e d the e x i s t e n c e of the m os t s t a bl e i nt erna l s t r u c t u r e of the m i x e d solvent. My own 1H -N MR s t u d i e s as well as the a n a l y s i s of t e m p e r a t u r e c o e f f i c i e n t s of e l e c t r i c p e r m i t t i v i t ies in the m i x t u r e s of Py - H 2 0 e n a b l e d me as well to d i s c l o s e the c o m p o s i t i o n range c h a r a c t e r i z e d by the s t r o n g e s t h y d r o g e n b o n d i n g b e t w e e n m o l e c u l e s of p y r i d i n e and water.
As I have m e n t i o n e d earlier, d e t e r m i n e d in this work val ues of the A S ( Py - H 20) s pe c t r a l p a r a m e t e r and the a c o e f f i c i e n t are g oo d c ri t e r i a for e s t i m a t i n g the m u t u al i n t e r a c t i o n s b e t w e e n
p ol ar c o m p o n e n t s of b i n ar y s ol v ent s. The A S ( P y - 1^0) v a l u es are v i s u a l i z e d in Fig. 2 as a f u n c t i o n of the m i x t u r e c o m p o s i t i o n s . They s ho w a d i s t i n c t m a x i m u m at ca. 33 mo l% of Py. As it was e v i d e n c e d e a r l i e r [1-4] the m a x i m u m for this p a r a m e t e r p o i nt s at the c o m p o s i t i o n s w he re the s t r o n g e s t i n t e r a c t i o n s b e t w e e n w at er and p y r i d i n e occur, and s t a b l e " co m p l e x e s " ( s u b - u n i t s ) Py • 21^0 are formed. Also the a v a l u e s reach m a x i m u m for the c o m p o s i t i o n h a v i n g ca. 33 m o l % of P y ( s e e Fig. 2). Further i n t e r e s t i n g r e s ul ts can be o b t a i n e d by d e t a i l e d a na l y s i s of the f u n c t i o n oi = f (mol% of Py). It i n d i c a t e s that on i n c r e a s i n g a d d i t i o n of Py to water up to 6 m ol % of Py, w h e r e a m i n i m u m is reached, c a u s e s a rapid dro p in the a values. F u r t h e r a d d i t i o n of p y r i d i n e to the s t u d i e d m i x t u r e s c a u s e s an i nc r e a s e in a , w h i c h r e s u l t s in a m a x i m u m at ca. 33 mo l% of Py. Th e re fo r e, it is p o s s i b l e to assume that sma ll a mo u n t s of p y r i d i n e b ei ng a dd ed to neat w a t er b re ak its i nt erna l s t r u c t u r e ( c o m p o s i t i o n r e g i o n up to 6 m ol % of Py), w h e r e a s f u r th er a d d i t i o n of py r id in e , up to ca. 33 m o l % of Py, m a k es the i nt e rn a l s t r u c t u r e of the m i x e d s o l v e n t m or e and more s t a b i l i z e d by the h y d r o g e n b o n d i n g b on ds b e t w e e n m o l e c u l s of its c o m p o n e n t s . The same a n a l y s i s ma de for the p y r i d i n e - r i c h c o m p o s i t i o n r e g i on s h o w s a p e r m a m e n t i n c r e a s e of v a l u es of the oi
c o e f f i c i e n t down to ca. 33 m ol % of Py. T he r efo re, it s ee ms that m o l e c u l e s of w at er in the s t u d i e d mi x tu re , w i t h i n the composition ra nge b e t w e e n 100 to 33 m ol % of Py, play a role of " s t r u c t u r e - - ma k er s " in r e s p e c t to m o l e c u l e s of Py in the neat sol vent . On the b a s i s of the p r e s e n t r e s u l t s and those fou nd in the l i t e r a ture, it may be c o n c l u d e d that the m os t s t a bl e i nt e r n a l structure of the b i n ar y s o l v e n t u nd er s t u d y o c c u r s at ca. 33 m ol % of Py, w h i c h c o r r e s p o n d s to f o r m a t i o n of the most s t a bl e " c o m p le x " (sub- - un its) of the Py • 2 ^ 0 - type, m o s t l y s t a b i l i z e d by the int er- m o l e c u l a r h y d r o g e n bonding. R E F E R E N C E S [1] C. M. K i n a r t, W. J. K i n a r t, L. S k u 1 s k i, Pol. J. Chem. , 59, 591 (1905). [2] C. M. K i n a r t, W. J. K i n a r t, L. S k u 1 s k i , Pol. J. C h e m . , 59, 599 (1905).
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W M I E S Z A N I N A C H P I R Y D Y N Y Z WODA METODĄ. P O M I A R U ICH W I D M H-N MR I P R Z E N I K A L N O Ś C I E L E K T R Y C Z N E J Z m i e r z o n o w id ma H -N MR c i e k ł y c h m i e s z a n i n wody i p i r y d y n y (Py) w t e m p e r a t u r z e 298 ,15 K, w z a k r e s i e od 2,42 do 66,79 mol% Py. Z d a n y c h d o ś w i a d c z a l n y c h w y z n a c z o n o p r z e b i e g i w z g l ę d n y c h p r z e s u n i ę ć c h e m i c z n y c h , Ś ( P y - H 2 0), p o m i ę d z y ś ro d k a m i s y g n a ł ó w p r o t o n ó w p o c h o d z ą c y c h od w od y i ś ro d k a m i , s y g n a ł ó w p o c h o d z ą c y c h od p i ę ci u p r o t o n ó w m o l e k u ł pir ydyn y. N a s t ę p n i e w y z n a c z o n o w a r t o ści p a r a m e t r u s pe k t r a l n e g o , A S ( P y - H 2 0), w b a d a n y c h m i e s z a n i nac h . Z t e m p e r a t u r o w y c h z a l e ż n o ś c i p r z e n i k a l n o ś c i e l e k t r y c z n e j w y z n a c z o n o dla b a d a n y c h u k ł a d ó w w ar t o ś c i t e m p e r a t u r o w e g o współczyn nika p r z e n i k a l n o ś c i e l e k t r y c z n e j cx. O t r z y m a n e w y n i ki w s k a z u j ą na t w o r z e n i e się w z g l ę d n i e t r w a łych " k o m p l e k s ó w " o s k ł a d z i e Py • 2 H 2 0 w c i e k ł y c h m i e s z a n i n a c h wod y i pir ydyn y.
