Adsorption of 2-mercaptopyridine and 2-mercaptopyrimidine on a silver colloidal surface investigated by Raman spectroscopy

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 441 (1998) 63-76

Adsorption of 2-mercaptopyridine and 2-mercaptopyrimidine on a silver colloidal surface investigated by Raman spectroscopy Yoon Soo Pang, Hyun Jin Hwang I, Myung Soo Kim* Department of Chemist~ and Center for Molecular Catalysis, Seoul National Universi~, Seoul 151-742, South Korea

Received 9 June 1997; accepted 17 July 1997

Abstract Adsorption of 2-mercaptopyridine (2MP) and 2-mercaptopyrimidine (2MPM) on a silver colloidal surface has been investigated over a wide range of solution pH by surface-enhanced Raman scattering. Even though these molecules take various forms in aqueous solution, depending on the bulk pH, 2MP and 2MPM adsorb on the silver surface mostly as their thiolate forms through the A g - S interaction. Spectacular lowering of the pK2 values of 2MP and 2MPM on the surface suggests that the nitrogen-silver interaction also plays a secondary role in the adsorption process. 2MP was found to take a more or less perpendicular or tilted stance with respect to the surface, whereas the N-protonation changed the orientation to the flat one. In the case of 2MPM, a flat orientation seems to be favored regardless of the N-protonation. © 1998 Elsevier Science B.V. Keywords: 2-Mercaptopyridine; 2-Mercaptopyrimidine; Surface enhanced Raman scattering

1. Introduction Surface-enhanced Raman scattering (SERS) [ 1 - 4 ] has been established as a useful technique for the spectroscopic investigation of a d s o r b a t e - m e t a l interaction. The detailed analyses of spectral features, such as relative intensities of vibrational bands and their shifts and broadening upon surface adsorption, provide valuable information on the surface adsorption mechanism [5-7]. Adsorption of mercaptans on silver surfaces has been a focus of substantial research interest [8-13]. It is well known that mercaptans adsorb mainly via the sulfur atom of the mercapto group. Recently, we

* Corresponding author. t Permanent address: Department of Chemistry, Kyunghee University, Seoul 130-701, South Korea.

investigated SERS of 4-mercaptopyridine (4MP) which possesses more than one atom or group which may function as a ligand in the surface adsorption and may compete for interaction with the surface [14]. Acquisition of high quality SER spectra enabled an extensive study on the spectral change with the bulk pH variation and allowed detailed spectral analysis. It was found that 4 M P adsorbs on the silver sol surface mainly via A g - S interaction and the nitrogen atom of the pyridine ring also plays a secondary role in the adsorption process. 2-Mercaptopyridine (2MP) and 2-mercaptopyrimidine (2MPM) also possess several groups which may function as ligands in the surface adsorption, i.e. a sulfur atom, one or two nitrogen atoms in the pyridine and pyrimidine rings, and an aromatic 7r system. Compared with 4MP, heteroatoms in 2MP and 2MPM are located in close proximity, resulting in an adsorption

0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0022-2860(97)00280-9

64

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

I ~

" H+ 4

I

I

ID

SH PK1 =-1.38

I

S PK2 = 9.81

I

H

H

H

H

I

II

IV

V

VIII

II III

.

I

vx

,H

H

'

I

I vxi

Fig. 1. Structural change of 2MP in aqueous solution with variation in pH.

mechanism which is different from the case of 4MP. In fact, as derivatives of SERS prototype molecules, pyridine and pyrimidine, SERS of 2MP and 2MPM have been reported already [15,16]. However, the SER spectra reported were not of a very high quality, and detailed spectral analysis was not reported. This paper reports the ordinary and SER spectroscopic investigation of these molecules. Focus is placed on the mechanism of adsorption of these molecules on the silver colloidal surface.

2. E x p e r i m e n t a l

The method of silver sol preparation was reported previously [17]. A small amount (20 izl) of 1 × 10 -4 M 2MP and 2MPM aqueous solution was added to 1 mL silver sol solution to obtain the final concentration of 2 x 10 -6 M. When needed, a small amount (10-20 ~1) of 5 x 10 -2 M BaC12 solution was added to obtain the final C1- concentration of 1 × 10-3~2 × 10 -3 M. After the sol solution had changed color from yellow to dark green, poly(vinylpyrrolidone) (PVP, MW 40 000) was added as a colloid stabilizer. The pH of the solution was adjusted with NaOH o r H2504 as needed and was measured using a glass electrode. A sample was irradiated with 514.5 nm radiation from an argon ion laser (Spectra Physics model 16406). The laser power was 60 mW at the sample

position and a glass capillary was used as the ampling device. Raman scattering was collected at 90 ° using an f/1.4 camera lens with 50 mm focal length and focused onto the entrance slit of a Spex 1877E Triplemate spectrometer equipped with a charge-coupled device. Slit widths were set at the spectral resolution of - 1 0 c m -~. Spex DM3000R software was used for data acquisition and processing. 2MP, 2MPM, 2-chloropyridine (2CP), 2-chloropyridine hydrochloride (2CP.HC1), and 2-chloropyrimidine (2CPM) were purchased from Aldrich and purified further by recrystallization in water when needed. All the chemicals otherwise specified were reagent grade and triply distilled water was used throughout.

3. R e s u l t s a n d d i s c u s s i o n

3.1. 2MP

Thioi-thione tautomerism of 2MP has been widely investigated both experimentally and theoretically [ 1832]. Thiol tautomer (see structure VI in Fig. 1) exists dominantly in dilute solution of non-polar solvents, as well as in the vapor phase. In contrast, 2MP exists as a thione tautomer (structure V) in an aqueous solution at neutral pH. Thione predominance in such polar solvents as water is affected significantly by association

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

65

# Q (~ ~r

(c) pH 12

J

>,, I

m

E. E. m

(~ r~

(b) pH 4.2

j

? o

(M O

I ,~

,

I 500

,

I 1000

~

,

I 1500

(a) in HCI

//

I 2500

,

I

3000

Raman Shift (cm 1) Fig. 2. OR spectra of 2MP (a) in concentrated HCI solution and in aqueous solutions at pH (b) 4.2 and (c) 12.

66

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

Table 1 Spectral data and vibrational assignments for 2MP"

Acidic

Vibrational assignmenff

SERS

ORS Neutral

Basic

Without CI h

With CI pH 9.4

pH 0.8 242

300 433 625

314 446 621

727 1012 1045 1094 1110 1145 1168

732 998 1036 1088

1251 1292 1391 1428 1461 1533

1614

3044 3099

1141 1168 1180 1237 1260 1375 1436 1449 1512

1589 1621 2510 3100

~(Ag-CI) ~(CS) ~(CS)/6all 6b

327 444

343 434

343 435

631 727 995 1051 1088

636 719 1005 1053 1085

635 718 1005 1052 1084

434 622 } 636 718 1006 1052 1084

1131 1155 1168 1224 1260 1273

1118 1154

1117 1153

1122 1154

9a 15

1234

1234

1282

1281 1375

1235 1266 1281 1375

~(CH) 3 14

1414 1452

1418 1455

1417 1454

1549 1584

1554 1580

1553 1579

3062 3110

3059 3109

3061 3110

1417 1446 1516 1553 1585 / 1605 /

u(CS)/6al 1 18aor 12 19b/u(CS)

19b 19a 8b 8a 2B(CH) J u(CH)

a Frequencies in cm '. Spectral features do not change with pH variation in the range of I 8. Taken from Ref. [39]. Ring modes are denoted in terms of Wilson notation: u = symmetric stretching,/3 = in-plane bending. d Overtone. of two thione m o n o m e r s to form a stable d i m e r (structure V I I ) [18,19,22,32]. In fact, 2 M P exists as the c e n t r o - s y m m e t r i c dimeric form (structure V I I ) in the solid phase [33]. Structures of the cation and anion of 2MP, f o r m e d by protonation and deprotonation respectively, were also investigated by U V / V I S spectroscopy [20,21]. In the case of the anion, the thiolate f o r m (structure IX) was found to be m o r e important than the thione form (structure V I I I ) . The structure of the cation cannot be explained with a single L e w i s structure either, because the tautomeric structure I I I is also possible in addition to the two resonance structures I and II. The thiol tautomer (structure II) is k n o w n to be less

important than the two thione tautomers (structures I and I I I ) by U V / V I S spectroscopic study [21 ]. Also, of these two tautomeric structures, the thione tautomer which has one H atom attached to the N atom (structure I) is likely to be m o r e important. The ordinary R a m a n (OR) spectra of 2 M P in concentrated HC1 and in aqueous solutions at pH 4.2 and 12 are shown in Fig. 2 ( a - c ) respectively. Since p K values o f 2 M P are - 1 . 3 8 and 9.81 (some authors suggested s o m e w h a t different values, - 1 . 0 7 and 9.97) [20,21], the three O R spectra in Fig. 2 ( a - c ) correspond to the R a m a n spectra o f structures I, V (or VII), and I X respectively, according to the above explanation, e v e n though the contributions f r o m the

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

resonance structures forbid assignment of a single Lewis structure to each spectrum [20,21]. The S - H stretching mode, ~,(SH), appears usually at --2570 cm -~ in the vibrational spectra of mercaptans. The only peak in this spectral region in Fig. 2(b) is the one at 2510 cm -I, which is likely to be the first overtone of the band at 1260cm -l. Then, complete absence of the u(SH) band supports the predominance of the thione form over the thiol form at neutral pH [34-36]. The same mode appears as a broad band at 2300-2550 cm -l in the cation spectrum, Fig. 2(a). The red-shift and broadening of the band indicates that the S - H bond takes part in hydrogen bonding as in the case of 4MP reported by Spinner [37]. The usual approach in the vibrational assignment for mercaptans is to refer to the spectra for the corresponding chloro compounds, because the frequencies of the same type vibrations in these compounds are very similar [35,36,38]. The OR spectra of 2CP and 2CP.HC1 obtained in this work are well matched with those reported by Spinner [39] and by Green et al. [40] and will not be reproduced here. The vibrational assignments for 2MP based on those for 2CP and 2CP.HC1 by Spinner and by Green et al. are summarized in Table 1. Assignments made for 2MP in the previous vibrational studies have also been taken into consideration [41,42]. The frequencies for the cation form of 2MP in Fig. 2(a), which are summarized in Table 1, agree with the corresponding values in the OR spectrum of 2CP.HC1. For example, the major bands at 433, 625, 727, 1012, 1045, !145, and 1614 cm -1 in Fig. 2(a) find their counterparts at 441, 622, 736, 1006, 1044, 1150, and 1612cm -1 in the spectrum of 2CP.HCI. 2CP-HC1 is expected to have structure II (See Fig. 1) with the SH group replaced by C1-. Then, the excellent spectral correlation mentioned above means either that structure II of 2MP is more important than thought previously, or that the resonance does not affect the vibrational frequencies noticeably. The frequency correlation between the anion form of 2MP and 2CP is also good: bands at 444, 631,727,995, 1051, 1131, and 1584 cm -j in Fig. 2(c) finding their counterparts at 427, 619, 726, 993, 1047, 1120, and 1580 cm -~ in the latter. Most of the major bands in the OR spectra of 2CP and 2CP.HC1 find their counterparts in the OR spectrum of the neutral form of 2MP (Fig. 2(b)). The most intense in the OR spectrum of the neutral 2MP is the band at

67

1260 cm -~, which does not find its counterpart in the spectra of 2CP and 2CP.HCI. The same band was observed as the most intense peak in the OR spectrum of the solid state and assigned to the CH in-plane bending [41,42]. Also interesting in the neutral OR spectrum of 2MP is the appearance of two bands assignable to the ring mode 8a, namely at 1589 and 1621 cm -~. In the vibrational spectroscopy of pyridine and its derivatives, appearance of the 8a mode at 1590 and --1620 cm -I is taken as an evidence for the unprotonation (see Fig. 2(c)) and protonation (see Fig. 2(a)) of the nitrogen atom [14,43,44]. It may be tempting to interpret the dominance of the 1589 cm -~ peak as evidence for the predominant presence of the thiol form (structure VI), contrary to the conclusion from the previous UV spectroscopic studies. The above interpretation does not seem to be valid, however, because the v(SH) band is not observed in the same spectrum. It is to be noted that the band at 1589 cm -1 is broader than the others, especially the corresponding one at 1584 cm -~ in the anion spectrum (Fig. 2(c)). This suggests that the band is due to the ring mode 8a of the thione dimer (structure VII). Participation of the hydrogen atom in the hydrogen bonding may be responsible for the red-shift (from 1620 cm -~) and broadening of this band. Then, the band at 1621 cm -1 may be assignable to the 8a mode of the monomer (structure V). The relative intensities of these two bands suggest that the dimeric form is more abundant, as expected from the large equilibrium constant reported in the literature [18,19,22]. An alternative explanation for this pair of bands is to assign them to two 8a modes of the dimer. Either way, the dimeric form seems to be dominant in the aqueous solution. The spectacular intensity of the band at 1260 cm i seems to be related to the formation of the dimer also. In our previous study on 4MP, the ring mode 9a was taken as another marker band for the N-protonation. A similar band appears at 1145 cm -1, 1141 cm -~, and 1131 cm I in Fig. 2(a-c) respectively. The ring mode 6b, appearing at 625cm i 621 cm -~, and 631 cm -~ in the cation, neutral, and anion spectra respectively, is found to be a useful N-protonation marker also (to be described later). The ~,(CH) pairs at 3062 and 3110 cm -I also change their frequencies to 3044 and 3099 cm 1 upon N-protonation. The frequency of the ring mode 1 also changes with pH

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

68

// o o

Q0~ ,~

~o'J

o~

~"

(f) pH 9.4

o

(e) pH 3.4 t~

Y

tt~

o e0e~

"

~

1

~, '~

t'~ "

~

#'D ¢~ CO

~~ ,

(d) pH 2.2

Y

(c) pH 1.5

J

~.J

¢1) ¢-

(D

O

ti n

,.~

~ . ~ o~

,,, ~,

,=.,==....-.,..,..

o

t'~ ¢.o

O

-'~ CD t',D ~

~1"

~

¢,o

(b) pH 0.8

O

,~ r.0 I

500

(a) without CI-

rid ~

,

I

1000

~

i

1500

,/I II

I

2500

,

I

3000

Raman Shift (cm 1) Fig. 3. SER spectra of 2 x 10 -6 M 2MP in silver sol (a) without C]- and (b-f) with 1 x 10 -3 M CI-. The pH of the aggregated sol solution is (a) 9, (b) 0.8, (c) 1.5, (d) 2.2, (e) 3.4, and (f) 9.4.

E S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

variation but will not be taken as the N-protonation marker as for the case of 4MP. The SER spectra of 2MP obtained with and without CI- in the sol solution are shown in Fig. 3. Without CI-, the SER spectrum was invariant with the solution pH over 1-9. On the other hand, a noticeable spectral change was observed with pH variation in the presence of C1 . First to be noticed in the SER spectra is the fact that no peak is present near 2570 cm -l, which is assignable to v(SH). Absence of this band and other spectral features in the SER spectra of various mercaptans led to the conclusion that mercaptans adsorb as thiolate forms via their sulfur atoms after losing thiol protons [14,34-36]. In this regard, it is important to note that the SER spectra without CIobtained over the entire pH range covered (Fig. 3(a)) and the SER spectra with C1- at medium acidic to basic condition (Fig. 3(e,f)) are better correlated with the anion OR spectrum (Fig. 2(c)) than others. For example, the marker bands mentioned above which appear at 635, 1117, 1579, 3061, and 3110cm -~ in Fig. 3(e,f) are better correlated with those at 631, 1131, 1584, 3062, and 3110cm -~ in Fig. 2(c) than those at 625, 1145, 1614, 3044, and 3099 cm -I in Fig. 2(a). This means that the species responsible for the SER spectra in Fig. 3(e,f) (Fig. 3(a) also) is an N-deprotonated form of 2MP. Also important is the fact that the ring modes containing CS stretching vibration such as v(CS)/6alI and v(CS)/ 6aI, which appear at 444 and 727 cm -I respectively, in the anion OR spectrum are red-shifted to 434 cm and 719 cm -~ in the SER spectrum, indicating the involvement of the sulfur atom in the adsorption process. The spectral features described so far suggest that 2MP adsorbs on the silver surface as the N-deprotonated thiolate form (structure IX) and its sulfur atom participates in the surface binding. As the solution pH is lowered, subtle changes occur in the SER spectrum obtained with C1-. The most readily observable of these is the appearance of another Vsa mode at 1605 cm -1, in the spectrum obtained at highly acidic condition (Fig. 3(b)), which is an N-protonation marker band. The 9a mode at 1117 cm -l in Fig. 3(f) also blue-shifts to 1122 cm -t in Fig. 3(b). The appearance of the Nprotonation band is also apparent for the 6b mode in the enlarged spectra shown in Fig. 4. N-protonation of the adsorbed thiolate would result in the structure IV.

69

It is to be emphasized that the N-protonation was not observed without C1- over the pH range covered (at p H i 0 or lower, SER spectrum could not be obtained). This may look surprising, considering that the pK value for N-deprotonation (pK2) is 9.81. However, dramatic lowering of pK value is frequently observed for surface adsorbates, which arises due to surfaceadsorbate interaction [14,44]. For example, pK2 of 4MP was found to decrease from 8.83 in an aqueous solution to ~3 on the silver surface in the previous study, which was taken as evidence for the participation of the nitrogen atom in the adsorption process. A similar explanation may be applicable in the case of 2MP also, even though more dramatic lowering of pK2 would have to be assumed. In the case of 4MP, the presence of C1- on the surface was observed to increase the pK2 value slightly (by 0.5-1.0), which was attributed to the well-known formation of a pyridinium-chloride ion pair on the surface [4547]. Assuming a similar difference in the pK value for N-deprotonation in 2MP between the cases with and without C1 , the absence of N-protonated bands in the latter case can be understood. Other evidence for the participation of the nitrogen atom in the adsorption process is found in the blue-shift of the ~,j mode at 1005 cm -1 in SERS from 995 cm -1 in the anion OR spectrum (Fig. 2(c)). Such a blue-shift of the ~,l mode is well-known in the surface adsorption of pyridine [44,48-50]. In the case of 4MP, adsorption via both sulfur and nitrogen atoms requires a flat stance of the pyridine ring with respect to the surface. Such a geometrical requirement means that the maximum overlaps between S and Ag and between N and Ag orbitals would not be achieved and the orientation may not be optimal for both of the bonds. With the sulfur atom located near the nitrogen atom in 2MP, such a severe geometrical requirement is not needed, and both the S - A g and N - A g bonds are likely to be stronger than those for 4MP. This seems to be the reason why lowering of pK2 is more dramatic for 2MP than for 4MP. When an aromatic mercaptan adsorbs on the silver surface via its sulfur atom, the aromatic ring is known to assume a tilted stance with respect to the surface plane [34,35,51-53]. On the other hand, surface adsorption of pyridine via its nitrogen atom favors a more or less perpendicular orientation of its molecular plane [48,50]. Surface orientation of 2MP will be

70

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

(f) pH 9.4

to

(e) pH 3.4

(d) pH 2.2 (/) tr-

g

(c) pH 1.5

(b) pH 0.8

to

to

~

to

to

(a) w

i

tCf h

o

I 600

u

~

,

I 650

Raman Shift (cm 1) Fig. 4. The ring mode 6b region of the SER spectra in Fig. 3. See the caption of Fig. 3. affected by both of these factors, a slightly tilted stance being likely to be thermodynamically favorable. It is well known that u(CH) bands of an aromatic ring appear distinctly when the ring assumes a perpendicular stance with respect to the surface, whereas the bands disappear with a flat stance [54,55]. The distinct appearance of these bands in Fig. 3(a,f) is in agreement with the adsorption mechanism of 2MP described above. In the SER spectrum with C1obtained at highly acidic condition (e.g. Fig. 3(b)), the p(CH) bands are substantially weakened. Namely, the flat stance of the ring seems to be favorable for the optimal S - A g and ion-pair interactions.

3.2. 2 M P M

Pyrimidines substituted with an SH or OH group at the 2 or 4 positions are fundamentally tautomeric heterocyclic systems also, and many experimental and theoretical studies on these molecules have been reported [22-24,26,29,31,56,57]. Results from the present OR and SER investigation on 2MPM are analogous to those on 2MP and only a brief description will be provided, especially for the spectral features pertinent to the adsorption mechanism. According to the UV/VIS spectroscopic studies, structural changes in 2MPM are similar to those in

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

71

-H+ SH pK1 = 1.35

1

2

S-

S pK2 = 7.14

4

5

7

I 3

8

6

Fig. 5. Structural change of 2MPM in aqueous solution with variation in pH.

Table 2 Spectral data and vibrational assignments for 2MPM a ORS Solid

Acidic

321 351 407

323

479

475

627 750 985 1054 1097 1175 1226

631 754

1346 1477 1496 1572 1604

brational assignmen(

SERS

1069

1209 1344 1404 1473

1598 1617

Neutral

Basic

pH 2.9

pH 1.0

239

240

415

415

424

753 994

452 479 650 745 1001

454 480 655 746 1004

455 480 652 746 1003

1084 1184

1079 1161

1080 1162

1080 1163

1243

1248

1247

1382

1378

1248 1339 1380

1574

1544 1563

351 423 467 478 636 748 987 1058 1098 1181 1218

Without CI- b With CI

1340

1378

1496

1586 1610

u(AgCl) ~(CS)

u(CS)/6alI 6b 6al/p(CS) 1 12 18b 15 ~(CH) 3 19a ~ 19b

/ 1562

1563 } 1586

8b 8a

Frequencies in cm i. b Spectral features do not change with pH variation in the range of 1-8. c Taken from Ref. [43]. Ring modes are denoted in terms of Wilson notation; u = symmetric stretching, ~3= in-plane bending.

72

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

//

(e) 2 C P M H C I ~

(d) pH 12

m m

I%

if)

E ~

to r~

f;O

QD ~ CD

(c) pH 4.7

E m

1%

o) (J~

~ 8

~ ~,

(b) in HCl

1%

°1 I

|

50O

•

•~ 1% to ~ o o ~

i

.

r.D

t'~ l,. ,~1. to

~

~ I

(a) solid ~ In

i

,

I

II

1000

1500

2500

3000

Raman Shift (cm 1) Fig. 6. OR spectra of 2MPM (a) in solid phase, (b) in concentrated HC1 solution, in aqueous solutions at pH (c) 4.7 and (d) 12, and (e) OR spectrum of 2CP.HCI in aqueous solution.

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76 Table 3 Spectral data and vibrational assignments for 2CPM and its hydrochloride (2CPM.HCI) a ORS

Vibrational assignment ~

2CPM-HC1

2CPM

332 397 448 455 631 756 999 1072 1104 1192 1217 1275

336 420 443 450 638 759 1002 1080 1107 1176

1569 1586 1600

~(CCI) 16a } 6a 6b 9a 1 12 18b 15 3 14 19a

1275 1391 } 1567

8a

Frequencies in cm t. b Taken from Refs. [60,61 ]. Ring modes are denoted in terms of Wilson notation; ~ = in-plane bending.

2MP, structures 1 and 5 (see Fig. 5) being the favored cation and neutral forms [22,56]. It is likely that the thione form (structure 5) dimerizes to the structure similar to VII in Fig. 1, even though no investigation on the formation of a dimer has been reported. No UV/VIS spectroscopic study on the anion structure has been reported either. In analogy with 2MP and 2-hydroxypyrimidine, it is thought that the thiolate form (structure 8 in Fig. 5) seems to be important [58]. The OR spectra of 2MPM in the solid state, in concentrated HC1, and in aqueous solutions at pH 4.7 and 12 are shown in Fig. 6(a-d). The presence of strong fluorescence backgrounds in these spectra necessitated their subtraction via numerical curve fitting. The OR spectra of 2CPM and its HC1 salt (2CPM.HC1) have been obtained also. The former is well matched with the spectra in the literature and is not reported here [59-61]. The OR spectrum of 2CPM.HC1, which has not been reported, is shown in Fig. 6(e). Since the pK values for deprotonation at the mercapto group and the protonated nitrogen are 1.35 and 7.14 respectively [56], structures 1, 5, and 8 are responsible for the spectra in Fig. 6(b-d) respectively. As in the case of 2MP, peaks appearing in the spectra have been assigned by

73

referring to previous vibrational spectroscopic studies on 2MPM and 2CPM [16,42,43,59-62], and also by utilizing the correlation with the OR spectrum of 2CPM.HC1 in the case of the cation form. The results are summarized in Tables 2 and 3. The ring mode 8a which served as an N-protonation marker band for 2MP shows a spectral change with pH variation in the case of 2MPM also. A distinct peak at 1574 cm -1 in the anion spectrum (Fig. 6(d)) is a clear indication of deprotonation of the nitrogen atoms. This peak appears as broad multiplets in the spectra of the neutral and cation forms, even though the presence of peaks at 1617 and 1610 cm -L in Fig. 6(b,c) suggests N-protonation. This peak appears as a doublet at 1572 and 1604 cm -j in the solid spectrum. This suggests that 2MPM exists as a dimer in the solid state, the same correlation being applicable to the neutral form in the aqueous solution (Fig. 6(c)). Also noteworthy is the intensity variation of the two ring-breathing modes 1,I and u 12. The u mode appears at 987 and 994 cm -1 in the neutral (Fig. 6(c)) and anion (Fig. 6(d)) spectra. It is completely missing in the cation spectrum (Fig. 6(b)), however. On the other hand, the v~2 mode appears distinctly at 1069 and 1058 cm -1 in the cation and neutral spectra and is absent in the anion spectrum. We do not have a clear explanation for their presence and absence, even though these bands can serve as structural marker bands. The appearance and disappearance of bands with pH variation is observed for several other bands also. The in-plane CH deformation at 1098 and 1084 c m -1 in the neutral and anion spectra disappears in the cation spectrum. The in-plane CH deformation at 1209 and 1218 cm -L in the cation and neutral spectra disappears in the anion spectrum. The ring v19a appears distinctly only in the anion spectrum at 1382cm -l, whereas the ring /)19b appears distinctly only in the neutral spectrum at 1496 cm -~. The band at 1340 cm -1 in the neutral spectrum, which could not be assigned, does not appear as a major band in the cation and anion spectra. Fig. 7 shows the SER spectra of 2MPM obtained in the absence of C1- at pH 8.0 (Fig. 7(a)) and in the presence of 2 x 10 -3 M CI- at pH 1.0, 1.6, and 2.9 (Fig. 7(b-d)). The spectra obtained without C1- did not display any spectral change with pH variation in the range 1.0 to 8.0. Also, the spectral change induced by the addition of CI- was not as significant as for

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

74

//

°

/

~

(d) pH 2.9

o

~

~=

~

(c) pH 1.6

>., i

m

(I) E.

(~ E. m

~o~ ~

~

(b) pH1.0

.~

I

I

500

I

(a) without CI-

I

1000

,

I

1500

, II

/I

I

2500

,

I

3000

Raman Shift (cm 1) Fig. 7. SER spectra of 2 x 10 -6 M 2MPM in silver sol (a) without C1- and ( b - d ) with 2 x 10 -3 M CI . The pH of the aggregated sol solution is (a) 8.0, (b) 1.0, (c) 1.6, and (d) 2.9.

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

2MP. Concerning the identification of the chemical species on the silver surface, the following spectral features are to be noted: the 8a mode appears distinctly at 1562cm-1; the ~'l mode appears at 1004 cm-~; the ~'12 mode is missing; the ~'18b mode appears at 1080cm-l; the in-plane CH bending appearing at 1209 cm -1 in Fig. 6(b) is missing; the p 19amode appears at 1378 cm-l; the ~,19bmode is missing; and, finally, the unidentified band near 1340 cm -l does not appear in the SER spectra except at pH 1.0 (Fig. 7(b)). All these are characteristic of structure 8 (see Fig. 5). Also to be noted is the fact that no peak appears near 2570 cm -I assignable to ~(SH), in agreement with structure 8. In analogy with the result from 2MP, the sulfur atom of the anion form (structure 8) is thought to be involved in the surface binding. One piece of evidence for its participation is the red-shift of the 6aI/~(CS) band at 753 cm -~ in the anion OR spectrum to 746 cm -~ in the SER spectrum. As the solution pH is lowered in the presence of CI-, new peaks begin to occur at 1339 and 1586 cm -~ (Fig. 7(b)) which may be considered as N-protonation markers. Hence, 2MPM is protonated in highly acidic media, even though the protonation is even more difficult than for 2MP. This suggests that the nitrogen atoms of 2MPM are involved in the surface binding. If only one of the two nitrogen atoms in 2MPM is involved in the surface binding, the situation will become similar to that for 2MP, and the molecular plane will assume a more or less perpendicular or tilted stance with respect to the surface. Then, the second nitrogen will be away from the surface and become available for protonation. The fact that the N-protonation of adsorbed 2MPM is extremely difficult suggests that the second nitrogen is also involved in the surface binding. Participation of the three atoms, one S and two N, in the surface binding would result in the flat orientation of the adsorbed 2MPM with respect to the surface, as was observed for 4MP. This, in turn, would result in the disappearance or substantial weakening of the u(CH) ring modes. It is interesting to note two tiny features at --3050 and - 3 1 2 0 c m -j in Fig. 7(a) and Fig. 7(d) which are assignable to these modes. We are not confident at the moment to take this as evidence for the perpendicular or tilted stance of the aromatic ring. Part of the reason is that the instrument used in the present work is equipped with a highly red-sensitive

75

detector (charge-coupled device) whose relative gain at this spectral range is at least an order of magnitude larger than the ordinary photomultiplier tube used previously [53]. Hence, based on the spectral features described above, we suggest that the adsorbed 2MPM lies fiat on the surface with three heteroatoms interacting with the surface. Even though the protonation of nitrogen atoms of the pyrimidine ring is very difficult, it can occur in highly acidic media when C1- is present. This suggests the presence of an ion-pair interaction between the protonated 2MPM and the C1- on the surface. Regardless of the protonation, however, these atoms or groups in 2MPM interact with the surface. This would favor the fiat stance of the pyrimidine ring with respect to the surface, as supported by the absence of the ~,(CH) bands in Fig. 7(b). In summary, the SER spectra of 2MP and 2MPM adsorbed on the silver colloidal particle surface have been obtained. While the molecules take the thione forms (structures V and VII in Fig. 1 and structure 5 in Fig. 5) in aqueous solutions at neutral pH, the surface adsorbates have been found to have the thiolate forms (structures IX and 8). Both the sulfur and nitrogen atoms contribute to the adsorption of 2MP, leading to a perpendicular or tilted stance of the pyridine ring with respect to the surface. Protonation of the adsorbed 2MP occurs at the nitrogen atom only in the presence of CI-, at much lower value of pH than pK2 in bulk solution. The orientation of 2MP in the presence of C1- changes to the fiat stance with respect to the surface upon lowering the solution pH. In the presence of C1- on the surface, the adsorbate takes the pyridinium thiolate form (structure IV) due to the stabilization via the ion-pair formation. Adsorption of 2MPM involves the interaction of a sulfur atom and two nitrogen atoms of the pyrimidine ring, resulting in a more or less fiat stance of the pyrimidine ring with respect to the surface. Protonation of the ring nitrogen atom of 2MPM occurs only in the presence of CI-, also at much lower pH than pK2 in bulk solution, resulting in the pyrimidinium thiolate form of adsorbed 2MPM (structure 4).

Acknowledgements This work was supported by the Basic Science Research Fund of Ministry of Education, Republic

76

Y.S. Pang et al./Journal of Molecular Structure 441 (1998) 63-76

of Korea and by the Specified Basic Research Fund of Korea Science and Engineering Foundation (KOSEF).

References [I] R.K. Chang, T. Furtak (Eds.), Surface Enhanced Raman Scattering, Plenum, New York, 1982. [2] S.M. Heard, F. Grieser, C.G. Barraclough, J.V. Sanders, J. Phys. Chem. 89 (1985) 389. [3] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Chem. Phys. Lett. 26 (1974) 163. [4] J.E. Pemberton, M.A. Bryant, R.L. Sobocinski, S.L. Joa, J. Phys. Chem. 96 (1992) 3776. [5] M.L. Patterson, M.,I. Weaver, ,i. Phys. Chem. 89 (1985) 1331. [6] T.H. Joo, K. Kim. M.S. Kim, ,i. Phys. Chem. 90 (1986) 5816. [7] M. Takahashi, H. Furukawa, M. Fujita, M. Ito, ,i. Phys. Chem. 91 (1987) 5940. [8] H. Sellers, A. Ulman, Y. Shnidman, J.E. Eilers, J. Am. Chem. Soc. 115 (1993) 9389. [9] R. Heinz, J.P. Rabe, Langmuir 11 (1995) 506. [10] M.M. Walczak, C. Chung, S.M. Stole, C.A. Widrig, M.D. Porter, J. Am. Chem. Soc. 113 (1991) 2370. [I 1] P.E. Laibinis, G.M. Whitesides, D.L. Allara, Y. Tao, A.N. Parikh, R.G. Nuzzo, J. Am. Chem. Soc. 113 (1991) 7152. [12] M.A. Bryant, .I.E. Pemberton, J. Am. Chem. Soc. 113 (1991) 8284. [13] S.B. Lee, K. Kim, M.S. Kim, W.S. Oh, Y.S. Lee, J. Mol. Struct. 296 (1993) 5. [14] H.S. Jung, K. Kim, M.S. Kim, J. Mol. Struct. 407 (1997) 139. [15] M. Takahashi, M. Fujita, M. lto. Surf. Sci. 158 (1985) 307. [16] W.H. Li, B.W. Mao, ZQ. Tian, J. Raman Spectrosc. 26 (1995) 233. [17] T.H. Joo, K. Kim, M.S. Kim, Chem. Phys. Lett. I 12 (1984) 65. [18] P. Beak, ,I.B. Covington, S.G. Smith, J. Am. Chem. Soc. 98 (1976) 8264. [19] P. Beak, Acc. Chem. Res. 10 (1977) 186. [20] R.A. Jones, A.R. Katritzky, J. Chem. Soc. (1958) 3610. [21] A. Albert, G.B. Barlin, J. Chem. Soc. (1959) 2384. [22] S. Stoyanov, I. Petkov, L. Antonov, T. Stoyanova, P. Karagiannidis, P. Aslanidis, Can. J. Chem. 68 (1990) 1482. [23] G.B. Barlin, D.J. Brown, M.D. Fenn, Aust. J. Chem. 37 (1984) 2391. [24] E. Spinner, J. Chem. Soc. (1960) 1237. [25] A.R. Katritzky, R.A. Jones, J. Chem. Soc. (1960) 2947. [26] L. Lapinski, M.,I. Nowak, J. Fulara, A. Les, L. Adamowicz, J. Phys. Chem. 96 (1992) 6250. [27] M.J. Cook, A.R. Katritzky, P. Linda, R.D. Tack, J. Chem. Soc. Perkin Trans. 2: (1972) 1295. [28] O.G. Parchment, N.A. Burton, I.H. Hillier, M.A. Vincent, J. Chem. Soc. Perkin Trans. 2: (1993) 861. [29] J.G. Contreras, J.B. Alderete, J. Mol. Struct. (Theochem) 231 (1991) 257.

[30] P. Beak, J.B. Covington, J.M. White, J. Org. Chem. 45 (1980) 1347. [31] M. Berndt, J.S. Kwiatkowski, J. Budzinski, B. Szczodrowska, Chem. Phys. Lett. 19 (1973) 246. [32] P. Peak, F.S. Fry, J. Lee, F. Steele, J. Am. Chem. Soc. 98 (1976) 171. [33] B.R. Penfold, Acta Crystallogr. 6 (1953) 707. [34] T.H. Joo, M.S. Kim, K. Kim, J. Raman Spectrosc. 18 (1987) 57. [35] H.M. Lee, M.S. Kim, K. Kim, Vib. Spectrosc. 6 (1994) 205. 136] H.M. Lee, K. Kim, M.S. Kim, ,i. Raman Spectrosc. 24 (1993) 661. [37] E. Spinner, J. Chem. Soc. (1962) 3127. [38] C.K. Kwon, M.S. Kim, K. Kim, J. Raman Spectrosc. 20 (1989) 575. [39] E. Spinner, ,i. Chem. Soc. (1963) 3860. [40] J.H.S. Green, W. Kynaston, H.M. Paisley, Spectrochim. Acta 19 (1963) 549. 1411 R. Shunmugam, D.N. Sathyanarayana, Spectrochim. Acta Part A: 40 (1984) 757. [42] D.N. Sathyanarayana, S.V. Kasmir Raja, Spectrochim. Acta Part A: 41 (1985) 809. [43] F.R. Dollish, W.G. Fateley, F.F. Bentley, Characteristic Raman Frequencies of Organic Compounds, Wiley, New York, 1974. [44] S.M. Park, K. Kim, M.S. Kim, J. Mol, Struct. 328 (1994) 169. S.M. Park, K. Kim, M.S. Kim, J. Mol. Struct. 344 (1995) 195. [45] T. Watanabe, O. Kawanami, K. Honda, Chem. Phys. Lett. 102 (1983) 565. [461 R.L. Birke, I. BErnard, L.A. Sanchez, J.R. Lombardi, ,i. Electroanal. Chem. 150 (1983) 447. [47] D.J. Rogers, S.D. Luck, D.E. Irish, D.A. Guzonas, G.F. Atkinson, J. Electroanal. Chem. 167 (1984) 237. [48] D.L. Jeanmaire, R.P. Van Duyne, J. Electroanal. Chem. 1 (1977) 84. [49] H. Ueba, S. Ichimura, H. Yamada, Surf. Sci. 119 (1982) 433. [50] M. Kobayashi, M. Imai, Surf. Sci. 158 (1985) 275. [51] T.G. Lee, K. Kim, M.S. Kim, J. Raman Spectrosc. 22 (1991) 339. [521 S.H. Cho, H.S. Han, D.J. Jang, K. Kim, M.S. Kim, ,i. Phys. Chem. 99 (1995) 10594. 153] S.H. Cho, Y.J. Lee, M.S. Kim, K. Kim, Vib. Spectrosc. 10 (1996) 26 I. [54] J.A. Creighton, Surf. Sci. 124 (1983) 209. [55] M. Moskovits, ,I.S. Suh, J. Phys. Chem. 88 (1984) 5526. [56] A. Albert, G.B. Barlin, ,i. Chem. Soc. (1962) 3129. [57] M.J. Nowak. H. Rostkowska, L. Lapinski, J. Leszczynski, J.S. Kwiatkowski, Spectrochim. Acta Part A: 47 (1991) 339. 158[ E. Spinner, J. Chem. Soc. (1960) 1232. [59] A..I. Lafaix, ,I.M. Lebas, Spectrochim. Acta Part A: 26 (1970) 1243. [60] Y.A. Sarma, Spectrochim. Acta Part A: 30 (1974) 1801. [61] S. Nakama, H. Shimada, R. Shimada, Bull. Chem. Soc. Jpn. 57 (1984) 2584. [621 R.K. Goel, C. Gupta, S.P. Gupta, Indian J. Pure Appl. Phys. 23 (1985) 344.