SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 51 (1995) 2177-2192
SERS of dithiocarbamates and xanthates Tse Yuen Koh Inorganic Chemistry Research Laboratories, Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2,4 Y, UK Received 16 May 1995; accepted 16 June 1995
Abstract
The surface-enhanced Raman spectra (SERS) of several simple dithiocarbamates and xanthates on silver colloids have been obtained. The dithiocarbamates studied are thought to adsorb with the -NCS2 moiety edge-on though the -OCS 2 groups of adsorbed xanthates are parallel to the surface.
1. Introduction Ligands containing the X-CS2 moiety, such as dithiocarbamates (X is R 2 N - ) and xanthates (X is R O - where R is an alkyl or aryl group), have an extensive coordination chemistry. Complexes of these so called 1,1-dithio ligands are known for virtually all transition metals and have been reviewed frequently [1-3]. Most of the spectroscopic data on dithiocarbamates and xanthates have been of the infrared only and normal force-field calculations of their complexes, in particular of the planar MS2CNC2 moiety, rather than of the free ligands themselves [4-7]. No surface enhanced Raman (SER) spectra have been reported for these ligands although sulphur donor ligands are well known to give good SER spectra, e.g. 1,1-dithiolato-2,2dicyanoethylene [8]. The SER spectra of sodium, potassium or ammonium salts of dimethyl- (Me2NCS~) (dmtc) and diethyldithiocarbamate (Et2NCS2) (detc); methyl- (MeOCS2-) (mexnt), ethyl(EtOCS2) (etxnt) and allylxanthate (CH2--CHCH2OCS~-) (alxnt) are examined here in detail and compared with the conventional Raman spectra of known complexes of some of these ligands. 2. Experimental Sodium dimethlydithiocarbamate dihydrate and sodium diethyldithiocarbamate trihydrate were obtained from Aldrich Ltd. The remaining dithiocarbamates and xanthates were made by literature methods [9,10]. For the former, the amines were added to carbon disulphide to form amorphous precipitates which were redissolved in either KOH or aqueous ammonia and left to stand until crystals formed. These were then filtered and washed with ether. Xanthates were made by adding carbon disulphide to a suspension of KOH in the appropriate alcohol dissolved in acetone, leaving the solution to stand until solids separated out. Present address: Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, Singapore 051 l, Singapore. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01510-8
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Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995)2177-2192
Platinum complexes of dimethyl and diethyldithiocarbamates ([Pt(dmtc)2] and [Pt(detc)2] were made by mixing aqueous solutions of Kz[PtCla] and the sodium salts of the ligands to yield green precipitates. Silver diethyldithiocarbamate [Ag6(detc)6] was made by adding aqueous AgNO3 to a solution of Na(detc) - 3H20; the yellow precipitate was recrystallised from pyridine [1 I]. Silver colloids were made by the citrate [12] reduction method and aggregated with MgC12 (1 drop of 2 M solution to 2 cm 3 colloid) to a green-grey colour where necessary. Raman spectra were measured using a Spex Ramalog 5 instrument with a Datamate data acquisition unit. Excitation radiation at wavelengths of 457.9, 488, 514.5 nm or 647.1 nm and powers of 50 to 100 mW were provided by Innova 70 Coherent Radiation argon and Innova 90 krypton lasers. A Perkin Elmer Series 1700X FT Raman spectrometer with a Spectron Laser Systems N d / Y A G laser (excitation at 1064 nm) was also used at powers between 200 mW and 2W. Spectra recorded with this instrument were corrected for instrumental response by multiplying by a correction function [13]. Infra-red spectra were measured on a Perkin Elmer 1720 infrared Fourier transform spectrometer; solids were measured as KBr discs. The enhancement factor (see tables) of a given band x in the SER spectrum relative to a reference band r is calculated using the formula /~.'(SERS)//r(SERS) /~-(normal Raman)/Ir(normal Raman)
3. Results and discussion 3.1. Dithiocarbamates ( R 2 N C S 2 )
Surprisingly there do not seem to be any reports of the vibrational spectra of free dithiocarbamate ions in the literature, so the assignments for the metal complexes have been used instead because the spectra are reasonably similar. An important structural difference between dithiocarbamates and xanthates is the amount of rc bonding between the carbon and the nitrogen/oxygen atom, i.e. the contribution of the resonance form (B) (Fig. 1), known as the thioureide form. This is manifested in the high frequency of the CN stretch at around 1500 cm-~ indicating the double bond character of the C=N linkage [14]. In contrast, the CO bond of the xanthates is not thought to have significant double bond character because the frequencies of the CO stretches are all below 1000 c m - ' [9,10]. Interest has also centered around the asymmetric CS2 stretch at around 1000 cm-~ in the infrared because the splitting of this band in dithiocarbamate complexes has been thought to indicate the denticity of the ligand, the so-called U g o - B o n a t i criterion [4]. This band, however, is not always strong in the Raman (if observed at all) and hence is not useful in interpreting the SER spectra. This criterion is also less applicable to the SER spectra because the SER bands are usually not narrow enough for such fine structure to be discerned. 3.1.1. Dimethyldithiocarbamate (dmtc)
The assignments given in Table 1 are based on those calculated for [Ni(dmtc)2] [5]. The bands in the Raman spectrum of this compound are fairly close in wavenumber to those
~
_ N
/
\=~s~
s
(A)
N÷
/
s-
(B)
Fig. 1. Resonance forms of dithiocarbamates.
Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995)2177-2192
2179
Table 1 Vibrational and SER spectra of dimethyldithiocarbamate (dmtc) (cm-~)
[Pt(dmtc)_,]
Na(dtmc) Raman Solid b
I.R./KBr
Raman
I.R./KBr
2954w 2923w
2914(8)
2907w
3012w 2928(2)
1486vs 1444sh 1395vw 1360vs 1240vs 1116vs 1043m 1019m 963vs 880w 849w 570w
1559(4) 1449(4)
1551vs 1441m
1550w 1448(4)
1.5 1.5
vCN, vsCHC, pCH 3 6~CH 3
1380(10)
2
1145(4)
4
946w 871w
1 1
6~CH 3 IJasCS2, pCS2, v~sCNC pCH 3, vsCS2 pCH3 v~CNC, v~CS 2 vsCNC, pCH3
572(4) 442(3)
0.5 0.5
vsCS2, vCN, vsCNC ,~CSS, 6CNC, v~CSM
329w
345(9)
348(2)
4
v~MS, vsMS
H20 soln ~
2960sh 2926(10) 2850w 1638(3) 1490sh 1440(6) 1395(3) 1369(10) 1250w 1122(3)
1496(1)p 1446(1)p 1404w 1374(5)p 1250vw 1132(1)p
961(3) 879(3)
970(1)p 882(1)p
573(10) 439(5)
576(10)p 440(5)p
334(1 ) 268( 1)
324(1 )p
1396(10) 1400vs 1243(1) 1243s 1151(2) l153vs 1054m 1007w 964(2) 963vs 898(2) 896w 572(9) 431(7)
568w 438m 364s 343w
SERS Ag colloid 647.1 nm
Relative Assignments ~ enhancement based on [Ni(dmtc)2] factor
v~CH v~CH
Assignments from Ref. [5]. b 20 = 1064 nm. ~ 2o = 647.1 nm.
of [Pt(dmtc)2]. SER spectra were recorded with various excitation wavelengths (Fig. 2) because it is known that the appearance of the spectrum may be simplified with excitations of longer wavelengths [15]. The origin of the "fluorescence background" (especially in Fig. 2, spectrum b) is unknown but is more likely a feature of the colloid system than the adsorbate. If the SER spectrum of dimethyldithiocarbamate obtained at 647.1 nm is compared with the Raman spectrum of the aqueous solution, it is found that the bands at 572 and 442 cm i are relatively weaker compared with the other bands. This effect is also found in the SER spectrum of diethyldithiocarbamate (vide infra) and is surprising because these are the symmetric CS2 stretch and bend which might be expected to be enhanced if the ion is binding to the silver surface by the sulphur atoms. However, bands due to the CH3 group are found with greater intensity. In the SER spectrum, the strongest band is that due to the symmetric CH3 deformation. The ratio of the intensity of the asymmetric/symmetric deformation (1448 and 1380cm -I, respectively) increases on going to near-infrared excitation. This behaviour is also found in the SER spectrum of diethyldithiocarbamate where the symmetric CH 3 deformation (1356 cm-') diminishes with respect to the asymmetric deformation (1456 cm-~). There are no substantial changes in wavenumber from the conventional to the SER spectrum except for the weak band at 946 cm -t (symmetric Me2N stretch) which shifts to lower wavenumber by 20 cm- ~ and the enhanced band at 348 cm- l which is thought to be due to an MS stretch. For [Pt(dmtc)z] also this occurs at 345 cm- ~ and is an intense peak. Of the other vibrations in the spectrum of [Pt(dmtc)2], the frequency of the C=N stretch is shifted most (by 60 cm-l). There is no firm evidence either way for the mode of orientation on the surface; the position of the C=N stretch is very similar to that in the free ion suggesting the preservation of the double bond character. This, however, also might indicate that the interaction with the metal is not very strong. The shifts in wavenumbers, though small, are still larger than those of diethyldithiocarbamate. This may be due to the smaller size
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Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995) 2177-2192
of the ion and the consequent greater perturbation felt on adsorption, as in the cases o f the SER spectra of methanol and p r o p a n o l [16]. 3.1.2. Diethyldithiocarbamate (detc) (Table 2, Fig. 3) T w o complexes of diethyldithiocarbamate, [Pt(detc)6] and [Ag6(detc)6], were prepared. The structures of these and Na(detc) are known by X-ray diffraction. In Na(detc), the diethyldithiocarbamate anion is ionically b o u n d to the sodium a t o m [17]; the complex [Pt(detc)2] is a square planar molecule with some interaction between the platinum a t o m and the CH2 group of the neighbouring molecule [18]. The complex [Ag6(detc)6 ] crystallises in two forms: the c~ form (used in this work) recrystallised from pyridine, reported by Yamaguchi et al. [11] and the fl form recrystallised from CSz at - 4 0 ° C , reported by Anacker-Eickhoff et al. [19]. In both forms o f [Ag6(detc)6] the structure of the ligand is the same, but there are three C=N bond lengths in the ~ form (1.328, 1.315 and 1.340/k) [11]. The S - C - S and M e - N - M e bond angles are, however, similar.
s
t
McJN (b)
3000
~W
25'oo 2600
' i
560
Wavenumber (crn -t) Fig. 2. Normal Raman and SER spectra of dimethyldithiocarbamate (dmtc): (a) Raman spectrum of sodium dimethyldithiocarbamate, saturated aqueous solution, 2o = 647.1 nm, 100mW; (b) SER spectrum of dimethyldithiocarbamate (0.5 x 10-~ M), silver colloid, 2o= 514.5 nm, 50 mW; (c) same as (b) but 2o = 647.1 nm, 100 roW; (d) same as (b) but 2o = 1064 nm, 150 mW.
~g
989(4)
912(2) 852(2) 785w
436(8)
344(9)
1004(2)p
910(2Rip 840vw 782w, dp
570(9)p
430(8)p
354w, p 306(4)p
559(10)
429(7) 396w 354(2) 312(2)
615w 562(8) 514w 453(1) 431(4) 400w 365(1) 318(2) 1.5 1.5 2 1.5
2
1.5 4 7 2
3 1.5 1.5
2 2
I 2
3 1 2
Enh. factor ~
432m 398m
974vs 904vs 836s 778m 610w 558s 506m
331(1)
433(3)
608(I) 566(4)
901w
1008(3)
1080(2)
1144(4)
1273(10)
1358(3)
1419vs 1384m 1354vs 1299rn 1267vs 1199vs 1140vs 1095m 1075s 1064s
1493(6) 1456(6)
2932
1064 nm
1489vs 1455m
2973s 2928s
I.R./KBr
3
2
2
1
3
2
3
4
5
3 3
Enh. factor d
Sers d Ag colloid
360w 330(1)
432(3)
1004(2) 968w 896(1) 832w 776w 608(I) 566(3) 500w
1080(2)
1272(10) 1200w 1142(5)
1376(1) 1356(2)
1490(5) 1454(3) 1426(2)
2934
647.1 nm
I 2
1.5
1
2 I 1 3 1
1
3
1
I 3.5
2 1 1.5
Enh. factor d
4 8
6
24 3
8 2 I
5
8 4 9
5 12
4.5 6 4
Enh. factor ~
6SCS vCS vCS 6CNC, 6NCH
6CSN
vSCS, vCS
vCC, 8CCH 6HCH, 8CCH 6HCH, vCS 6SCS, vCN vCS, 6SCS
vCS,6CCH vCS,6CCH
6NCH 6CCH, 6CCH, 8CNC, 8CCH,
8HCH, vCN 8CCH, 8HCH 8CCH, 8HCH
vCN, 8HCH 6HCH, 6CCH, vCN
Assignments based on [Zn(detc)2]2 [19]
20 = 1064 nm. b 2o = 514.5 nm. ~ Relative enhancement factor with respect to normal Raman spectrum of aqueous sodium diethyldithiocarbamate, d Relative enhancement factor with respect to normal Raman spectrum of aqueous sodium diethyldithiocarbamate, e Relative enhancement factor with respect to normal Raman spectrum of [Ag6(detc)6 ].
571(6)
1078(3)
1298(I) 1273(10) 1201(I) ! 144(5) 1095(2) 1079(3) 1066sh 1006(3) 973(3) 903(6) 840w
1372(2) 1357(2)
1377(3) 1355(1)
1300(2) 1284(9) 1204(I) I 159(3)
1493(10) 1458(5) 1430(5)
I 138(5)p 1096sh 1076(4)p
1302vw 1268(10)p
1486(9)p 1458(5)p 1438w, dp 1418(8)p 1382(2)p 1356(4)p
2973 2934
Raman ~
[Ad6(detc)6]
1538(10) 1528(8) 1453(5) 1436(5)
1133(4) 1092(2) 1076(5) 1065(2) 1005(4) 985(2) 911(2) 838w 778(I)
1477(8) 1452(9) 1436(5) 1416(5) 1371(4) 1360(2) 1345(I) 1300(2) 1264(10)
2982 2930
Raman solid ~
Raman solid ~
Raman soln/H,O b
[Pt(detc),]
Na(detc) • 3H_,O
Table 2 Vibrational and SER spectra of diethyldithocarbamate (detc) (cm -~)
-~ oo
-~' ~'" ~.
'~
Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995) 2177-2192
2182
Although the vibrational spectrum of the free ion itself has not been assigned, it can be safely assumed that all the bands above 900 c m - ' except the C=N stretch (1486 c m - I), the asymmetric Et2-N stretch (possibly at 1138 cm-i) and the symmetric CS2 stretch (possibly at 1004 cm-I) are due to C - H deformations coupled with C-C stretches. The assignments for the vibrational spectrum of the zinc dimer [Zn(detc)2]2 have been used because its spectrum is close to those of the free ion itself and of the silver hexamer [Ag6(detc)6 ] [20]. The C=N stretch in the Raman spectrum of solid Na(detc) • 3H20 at 1477 c m - 1 is high compared with that for the anhydrous compound [21]. In aqueous solution, the C=N stretch is even higher at 1486 cm-1; this has been attributed to hydrogen bonding between the sulphur atoms and water which is thought to strengthen the C=N bond slightly [21]. In [Ag6(detc)6] this stretch is barely higher, while in [Pt(detc)2] it is at 1538 cm-~. There are no other big changes although it is unusual that all the bands including that at 1000 c m - 1 supposedly due to the asymmetric CS2 stretch are polarized in aqueous solution. The frequency of the C---C stretch of ethylene decreases in the order: ethylene (gas) > ethylene (SERS) [22] >/[Ag(C2H4)] + [23] > K[Pt(C2H4)CI3] • H20 [24]. A corn-
[Ag6(detc)6] I
=1 =i
{b)
v
ou,l
i
l
l
i
3610.0
a ii
[ iliO.O
I glO
i US
i ~I0
i talO
~
i
I iO0
I ilO
i 4O0
Wavenumber (cm"l) Fig. 3. (a) SER spectrum of diethyldithiocarbamate (detc) (0.5 x 10 -3 M), silver colloid, 2o = 1064 nm, I00 roW; (b) Raman spectrum of [Ag6(detc)6], solid, 2o = 1064 nm, I00 roW.
i
am
Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995) 2177-2192
2183
parable ordering is also found albeit in the opposite direction in the frequency of the C=N stretch in diethyldithiocarbamate (detc) [Pt(detc)2] > [Ag6(detc)6]/>detc(SERS) >/ Na(detc). The increasing strength of the C=N bond may be conceived as the positive metal cation "attracting" the nitrogen lone pair electrons to "spread" out the charge of the complex, i.e. to favour the canonical form (B) in Fig. 1. The positions of other bands in the SER spectrum are closer to those in the conventional Raman spectrum of [Ag6(detc)6] than those of Na(detc), suggesting that the anion coordinates by the two sulphur atoms (although these differences are of the order of only about 10cm ~). The CH bends are clearly observed as are the CS stretches. The enhancement factors (relative to the free ligand) are also all of the same order. The relative enhancement factor calculated with respect to [Ag6(detc)6 ] is similarly uninformative. Of the two symmetric CH 3 deformations at 1376 and 1356cm -j, the latter seems to be enhanced in the SER spectrum compared to the normal Raman spectrum of the silver hexamer [Ag6(detc)6]. The latter band is nevertheless stronger in the solution Raman spectrum of the free ligand. Some bands are more enhanced in the SER spectrum, e.g. those at 608, 1004, 1142 and 1272 cm ~. The explanation that this is because these bands have some CS character is not entirely satisfactory because there are several other bands which are not particularly enhanced, e.g. the symmetric CS2 stretch itself at 566 cm ~. Of the CH stretches, only the methylene CH 2 stretch is found in the SER spectrum. Because the other CH stretches are strong in the conventional Raman spectrum of [Ag6(detc)6], it may be thought that this is so because the terminal methyl group is too far from the surface to be enhanced. The methyl group is, however, closer to the surface than the benzene ring in dibenzyldithiocarbamate (vide infra), in the SER spectrum of which the ring modes are very intense. In the SER spectra of 4-(4-nitrobenzyl-)- and 4-benzylpyridine on electrodes, the peaks due to the "chromophores", i.e. NO2 stretch and benzyl trigonal breathing mode, respectively, are strong and enhanced at long excitation wavelengths even though the nitro and benzyl groups do not interact directly with the metal surface and may be far from it if the pyridyl ring is edge-on [25]. However, Moskovits and Suh believe that SERS intensity decreases when a vibrational group is separated from the surface by several ~ngstr6ms, e.g. the CH stretches in the SER spectra of alkyl chain carboxylic acids (where there is no ~r bonding system [26] and phthalazine (where there is) [27]. Although it is more likely that the detc anion is adsorbed edge-on through the sulphur atoms, application of any selection rules [15] is limited because of the high degree of vibrational coupling and lack of proper assignments. It would be sterically impossible for the ion to adsorb flat-on due to the ethyl groups, assuming the surface is flat. It is conceivable that the nitrogen atom might itself bond to the metal, changing from an sp 2 to an "sp 3'' geometry so that the ethyl groups would be "lifted" out of the way (Fig. 4) as may indeed be the case for dimethyldithiocarbamate. This would however imply that the frequency of the C=N stretch should be lowered in wavenumber as is found in the SER spectra of alkenes. Evidence for this is not found in the SER spectrum; indeed the contrary is found, i.e. the CN stretch is slightly higher in wavenumber. 3.1.3. Other substituted dithiocarbamates (Table 3)
Dithiocarbamates with other substituents such as the morpholine, pyrrolidine, piperidine and benzyl group were prepared and SER spectra measured. Although assignments of the first three non-aromatic heterocyles are found in the literature, the spectra of the dithiocarbamates are quite different and cannot really be compared because of the mixing in of modes involving the CS2 group. The interpretation of the SER spectra is therefore much more complicated than the previous cases, especially because different conformations are possible for the non-aromatic heterocycles. No firm conclusions can be reached as yet concerning the orientations. In all cases, the C=N stretch appears to shift to higher wavenumber only slightly, the asymmetric CS2 stretch is usually not observed, while the asymmetric CS2 stretch is usually less prominent than in the two preceding dithiocarbamates. The SER spectra tend to be simpler than the conventional spectrum and weaker than those of the two preceding molecules.
3061
1592(4) 1493(1) 1442(1) 1312(4) 1218(1) 1006(10) 838(1) 571(I) 414(1)
3060
1598(10) 1506(4) 1440(2) 1354(5) 1204(4) 1004(10) 830w 538(5) 411(I)
2969 2870 1454(2) 1412(3) 1325(1) 1246(1) 1169(I) ll12w 1043(1) 1001(1) 919(I) 881(I) 704(1) 571vw 447(10) 1465(3) 1419(3) 1353(I) 1229(8) 1114(2) 1014(3) 875(2) 818(2) 542(10)
SERS "
1462(2) 1403(3) 1351(3) 1219(7) 1106(I) 1026(7) 870(1) 824(1) 545(10)
Raman b solid
Morpholine (mordce) O(C2 H4)2NCS~
2939 2856 1474(10) 1456(10) 1346(5) 1263(4) 1236(10) 1128(8) 1024(6) 952w 865(2) 804(2) 616(1) 550(4) 517(3) 406(2)
SERS"
Piperidine (pipdtc) CsH.~NCS~
2941 2856 1470(7) 1457(7) 1355(7) 1263(4) 1222(10) 1131(4) 1026(4) 969(2) 865(2) 812(3) 620(2) 571(1) 518(10) 400(9)
Raman b solid
1200(3) 1031(2) 1003(10) 804( 1) 621(I) 553(I)
3056
SERS "
Dibenzyl (dbtc) (C6HsCH2)2NCSf
1192(1) 1031(1) 1003( I 0) 796( I ) 627(1) 554(1)
3055
Raman b solid
" SERS on Ag-citrate colloids with 1064 nm excitation, b Normal Raman spectrum of solid salts; all potassium except ammonium for phenyldithiocarbamate. Note: All the SER bands are listed but not all the bands in the normal Raman spectrum.
2964 2869 1460(10) 1435(9) 1330(1) 1247(1) 1171(1) 1106(1) 1039(2) 993(1) 918(8) 872(2) 702(1) 557(2) 449(10)
SERS ~
Raman b solid
SERS ~
Raman b solid
Pyrrolidine (pyrdtc) C4HsNCS 2
Phenyl (phendtc) (C6Hs)HNCSf
Table 3 SER spectra of some substituted dithiocarbamates (cm-~)
t-,a
--,4
ixa
-2
IxJ m
Tse Yuen Koh/Spectrochimica Acta Part .4 51 (1995) 2177-2192
R~o.
2185
,...,.S
c~
s
Fig. 4. Possible orientations of dialkyldithiocarbamates. The SER spectrum of pyrrolidinedithiocarbamate closely resembles the normal Raman spectrum. There are wavenumber shifts of up to 10 cm-1, the relative intensities are mostly unchanged except the CN stretch at 1460 c m - l which is stronger. In dibenzyldithiocarbamate, the trigonal breathing bend of the benzene ring near 1000 cm-1 is clearly seen in the SER spectrum and also the CH breathing mode. Comparison of the normal Raman spectra of piperidine [28] and piperidinedithiocarbamate shows that in the strongest bands in the former (skeletal stretch at 810 cm ' and C H 2 scissors at 1449 cm i) are weaker in the latter; all the peaks in the 800 to 1500 cm-1 region are of roughly the same intensity. In the SER spectrum of piperidine on electrodes [29], the two bands mentioned before are much less intense compared with other bands due to C H 2 deformations at 1017, 1157, 1176 and 1257 cm ~and the skeletal stretch at 864 c m - i . In the SER spectrum of piperidinedithiocarbamate, however, there is no drastic change in the relative intensities. Three bands at 1167, 1071 and 1007 cm-1 do not appear in the SER spectrum at all even though the last two are quite strong in the normal Raman spectrum. The reason for this is not clear. The quality of the SER spectrum of the monosubstituted dithiocarbamate, PhHNCS2 unfortunately degrades within the space of approximately 1 min. This is probably due to hydrolysis of the ion by water [9,10] even though the bulk pH of a typical silver colloid used here was around 7. There is also the possibility that the surface charge of the colloid is responsible for the "disappearance" of the SER spectrum; however, in the absence of any electrochemical SERS data it is not possible to conclude this with certainty. 3.2. Xanthates ( R O C S j ) The vibrational spectra of xanthate complexes have come under less scrutiny although comprehensive assignments for some xanthate anions exist [30], which are used in this paper. It was not possible to obtain spectra of any xanthate complexes as they burn up even at very low laser powers. 3.2.1. Methylxanthate (Mexnt) (Table 4, Fig. 5) Like dimethyldithiocarbamate, the symmetric CS2 stretch is prominent in the SER spectrum of methylxanthate at 609 c m - 1 and diminished with respect to the other bands. It is, however, displaced farther downward in wavenumber as are the symmetric COC stretch at 931 cm-1 and the OCS2 out-of-plane deformation at 554 cm-1. The latter band is also much stronger in the SER spectrum suggesting that the OCS2 moiety is flat with respect to the surface. The weak band at 344 c m - ' can be assigned variously to a CS2 bend [30] or a MS stretch. Another point of difference with dimethyldithiocarbamate is that the CH vibrations are not so strong in the conventional Raman spectrum of methyl xanthate; the CH3 symmetric deformation at 1431 cm-1 is very much weaker than the symmetric CS2 stretch at 622 c m - l both in the SER and normal Raman spectra, while in the SER spectrum of dimethyldithiocarbamate the former is very much enhanced at the expense of the latter. The broad bumps between 1000 and 1200 c m - ' probably arise from the CH 3 rocking vibrations. In the CH stretch region, the three peaks due to the symmetric and asymmetric C H 3 stretches and the overtone of the C H deformation are little changed in position and in intensity relative to each other. Comparing these results with SER spectra of other
2186
Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995)2177-2192
Table 4 SER spectrum of methylxanthate (mexnt) (cm -~) Raman K(mexnt) solid
SERS Ag colloid 1064 nm
Relative enhancement factor
Assignments [30]
3012( l ) 2936(2) 2829w 1448(I) 1432(I)
3020w 2938(1) 2832w
I 1 1.5
1431(1) 1371w 1218w, br ll15w, br 1004w, br 931(4) 609(10) 554(4) 472(7) 344w
2
v,,~CH3 vsCH3 overtone ~CH~ 6~CH 3 6~,~CH~ 6~CH~ G.,COC pCH~, G~CS2 v~CS2, pCH 3 v~COC v~CS2 7OCS2 ¢5COC, rSOCS CS2 p CS2
1189(1) ll12w 1055(1/2) 944(2) 622(10) 583(1/2) 477(5) 336(3) 291(2)
3 1 6 1
molecules which contain methyl groups, it is found, for example in the SER spectrum of methanol where the CO bond is thought to make an angle of less than 30 ° to the surface, the asymmetric stretch at 2947 cm ~ is much stronger than the symmetric stretch at 2840 cm -~ (the former is more intense in the spectrum of bulk methanol) [16]. This situation is, however, complicated by the contribution of part of the intensity due to Fermi resonance with a CH deformation [14]. A different situation is found in the SER spectrum of acetonitrile where the C H 3 group is thought to interact with an adsorbed iodide ion with the CN group pointing away from and perpendicular to the surface. In this case only the symmetric C H 3 stretch is intense [31] as in the normal R a m a n spectrum of acetonitrile itself [28]. The symmetric CH 3 stretch is also found to be enhanced in the SER spectrum of toluene on gold electrodes [32]. It is thought that the ion adsorbs with the OCS2 face-on to the surface with the methyl group sticking out. The main evidence for this is the high intensity of the OCS2 out-of-plane wag. If the OCS2 moiety is face-on, there might be some bonding between the lone pair of the oxygen atom and the metal (Fig. 6). It would follow that the CO bonds would weaken and this does indeed seem to be the case because the COC stretch in the SER spectrum shifts by 15 cm-~, while the C=N stretches in the SER spectra of dithiocarbamates are virtually unchanged. In the SER spectrum of methanol on electrodes, the CO stretch shifts by 10cm ~ from 1038 to 1028 cm -~, although this shift is only observed for methanol and not for higher alcohols [16]. 3.2.2. Ethylxanthate (etxnt) (Table 5, Fig. 7) Some of the bands in the SER spectrum of this molecule are again shifted by 1 0 - 2 0 c m - l downwards, e.g. the two CCOC stretches (868 and 1014cm -~ shift to 858 and 1005cm -~) and the CS2 stretch (667cm - j shifts to 649cm-~). The OCS2 out-of-plane wag increases in intensity. As in the SER spectrum of the preceding ion, the asymmetric CH3 deformation and the CH 3 rocks are not very strong except, unexpectedly, for that at II13 cm -t. The c n 3 stretches do not feature at all; the strong band at 2932 cm -~ is assigned to the asymmetric CH2 stretch.
Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995) 2177-2192
2187
om
om t_
°,..~
r~
m
(a)
i! "!i
I,d
li
.
I 3500.0
I
.
i
.
r '
.
I
I
I
I
1
I
I
l
2000.0
1
t00.0
Wavenumber (cm1) Fig. 5. Normal Raman and SER spectra of methylxanthate: (a) Raman spectrum of potassium methylxanthate, solid, 2o = 1064 nm, 300 mW; (b) SER spectrum of methylxanthate (10 -4 M), silver colloid, 2o = 1064 nm, 300 mW.
An interesting feature of the SER spectrum is that two pairs of CS2 stretches (649 and 604 cm-~) and COC bends (500 and 450 cm-~) can be found. These are attributed to the presence of two conformers, trans and gauche. Separate bands are found in the conventional Raman spectrum of the sodium salt (671 and 625 cm-~; 492 and 454 cm- ]) [30] but only one set of bands in the spectrum of the solid potassium salt where only the trans conformer is present [30].
Me~
C.""'~"~aS
Fig. 6. Possible orientation of methylxanthate.
2188
Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995) 2177-2192
Table 5 SER spectrum of ethylxanthate (extnt) (cm-~) Raman K(etxnt) solid 2987w 2962(1) 2922(1 ) 2869w 1462(1) 1448w 1386w 1354w 1259w 1174w 1142w 1127w 1096( 1) 1062(4) 1014(1 ) 868(1) 814w 667(10) (625) ~ 583w (492) ~ 450(10) 404(4) 312(1 )
SERS Ag colloid 1064 nm
Relative enhancement factor
2935(5)
2.5
1447(2) 1389(1) 1358w
2 4
1113(4)
5
1005(2) 858(8)
2 4.5
649(3) 604(3) 552(2) 500( 1) 450(10) 404(4) 327(3)
Assignments [30]
v~CH 3 v~CH 3 v~CH 2 v~CHs 6a~CH 3, 6CH 2 /~a~CH~, 6CH, ~CH3, pwCH 2 6~CH 3, p,,,CH 2 pCH3, ptCH2 va~COC pCH3, v~CS 2 pCH~, ptCH2 va~CS2 vCCOC v~CCOC pCH 3, pCH 2 v~CS2 (trans conformer) v~CS2 (gauche conformer)
/OwOCS2 1
~COC 6COC 60CC 6CS 2
Found in the sodium salt only [30].
The proposed orientation is probably similar to that for methylxanthate (Fig. 6). The existence of two conformers in probably roughly equal proportions is not precluded by a face-on adsorption because there is probably little steric hindrance to rotation about the Cethyi--O bond. The weak normal Raman spectrum of Ag(etxnt) (the structure of this complex is unknown) shows only one strong band at 650 cm- 1 and three weak ones at 563, 453 and 416 cm-1; the position of the symmetric CS2 stretch is therefore close to that in the SER spectrum. The presence of the out-of-plane OCS2 deformation may suggest that the enhancement of this band in the SER spectrum is related to the binding to the metal rather than the proposed face-on orientation of this moiety.
3.2.3. Allylxanthate (Table 6, Fig. 8) No assignment of the vibrational spectrum of this ion has been reported although the bands associated with the olefinic group are very similar to those of the parent allyl alcohol [28]. It is the bands associated with the olefinic group which dominate the SER spectra, i.e. the C---C stretch at 1647 cm-% the symmetric CH2= bend at 1423 cm -1 and the -CH= bend at 1292 cm-', although the out-of-plane CH2= deformation at 944 cm-1 is weaker in the SER spectrum. The ion nevertheless probably does not adsorb by its olefinic group because the frequency of the C--C stretch is not shifted at all in contrast to results from the SER spectra of other olefins such as the various isomers of butenes [33,34] and ethylene [22] where the frequencies are shifted by about 50 cm- 1. As in ethylxanthate, the CS2 symmetric stretch in the SER spectrum is 10 cm -1 below that of the free ion and there appear to be two bands in this region, although the latter may be due to an OCS2 wag. The band due to the OCC bend at 410 cm-I is also fairly intense. The CO stretch at 902 cm-i is also shifted downwards in wavenumber by 15 cm -I.
2189
Tse Yuen Koh/Spectrochmffca Acta Part A 51 (1995)2177-2192
The absence of any methylene vibrations is puzzling because the CH 2- rock and wag at 1454 and 1339 cm 1 are at least half the intensity of the -CH= bend in the normal Raman spectrum. In the SER spectrum of l-butene on silver films, where the C=C bond must be parallel to the surface, the strongest bands are the C=C stretch at 1607 c m - J, the -CH--bend at 1296 cm -1 and the out-of-plane CH2= wag at 910 c m - ' . The first is shifted by 36 cm ~ while the other two bands are unshifted [33]. The symmetric CH2= deformation at 1415 cm -1, which is half the intensity of the C=C stretch in the normal Raman spectrum, is very weak in the SER spectrum. A possible explanation for the appearance of the SER spectrum of allylxanthate is that the ion is adsorbed in the manner depicted below (Fig. 9). In this configuration, the methylene CH bonds are almost parallel to the surface and the olefinic group projects out of the surface. If the OCS2 moiety were perpendicular to the surface, the methylene CH bonds could not be parallel to the surface. This configuration might explain why the CH 2 rock and wag are weak and also why the CH2= symmetric deformation is stronger than in the SER spectrum of 1-butene where this would be parallel to the surface [33]. Nevertheless this does not fully explain why the CH2 twist at 1225 cm t in the normal Raman spectrum of allylxanthate is not found in the SER spectrum; neither does it explain why both types of CH stretch are very much weaker than both the CH stretches of ethyl and methylxanthates together with the other olefinic CH bends and wag of allylxanthate itself.
Et o
(i-
,ill
i
M
Jl
(a)
j I m,
I
am
1 aooo,o
I
looo
I
looo
I
1,1oo
I
12oo
I
|ooo
1--
ooo
I
5oo
;
L/ I
,~o
Wavcnumber(cm-l)
Fig. 7. Normal Raman and SER spectra of ethylxanthate: (a) Raman spectrum of potassium ethylxanthate, solid, 2o = 1064 nm, 300 mW; (b) SER spectrum of ethylxanthate (10 -4 M), silver colloid, 20 = 1064 nm, 300 mW.
~0
100.0
2190
Tse Yuen Koh/Spectrochimica Acta Part A 51 (1995)2177-2192
,u
(b)
=i e~ t_. ¢¢
,j
(a)
I
m.O
I
•lm
l
I
I
llN
I
I
I
Wavenumber
I
I
D
~
I
I
40e
I
aO0
100.0
(cm "l)
Fig. 8. Normal R a m a n and SER spectra of allylxanthate: (a) R a m a n spectrum of potassium allylxanthate, solid, 2 o = 1064 nm, 500 roW; (b) SER spectrum of allylxanthate 0 0 -4 M), silver colloid, 2 o = 1064 nm, 300 roW.
4. Conclusions A comparison of the SERS of some simple dithiocarbamates and xanthates suggests that the modes of adsorption of" the two classes of compounds are different.
H
H
.,,,~on S
Fig. 9. Possible orientation of allylxanthate.
Tse Yuen Koh/Spectrochimica Acta Part .4 51 (1995)2177-2192
2191
Table 6 SER spectra of allyl xanthate (alxnt) (cm -~) Raman K(alxnt) solid
SERS Ag colloid 1064 nm
Assignments
3091 (3) 3013(7) 2990(4) 2963(2) 2936(5) 2879(2) 1647(8) 1454(2) 1423(3) 1339(3) 1290(7) 1225(4) 1155w 1094w 1049(2) 991(1) 968(2) 939(4) 916(5) 708(4) 611 (10) 469(4) 402(6) 376(2) 289(5)
3090w 3020w
v~CH2= vCH=
1647(10) 1423(7) 1292(4)
vasCH2 vsCH2 vC=C pCH 2 /i~CH2= p,~CH 2 6 CHptCH2
v~CS2
944(2) 902(7) 600(2)
trans pwCH2 = pwCH2= vCO v~CS2 6COC
410(8)
Acknowledgements The author would like to thank Professor W.P. Griffith, Imperial College, for helpful comments and advice, the University of London Intercollegiate Research Service for the Raman spectrometers at Imperial College, the Department of Education for an Overseas Research Studentship and the Koh-Sng Foundation for a grant.
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