Vibrational analysis of alkyl xanthates

Specrrochimica Acta. Vol Printed m Great Britain.

43A. No. 3. pp. 317-322.

05X4-8539/87 13 00 + 0.00 Pergamon Journals Ltd

1987

Vibrational analysis of alkyl xantbates N. B. COLTHUP and L. PORTER POWELL* American Cyanamid Company, Chemical Research Division, Stamford, Connecticut, U.S.A. (Received 26 December 1985; accepted 28 May 1986)

Abstract-Rotational isomers are indicated in the Raman spectra of sodium ethyl and other xanthates. Vibrational bands useful for characterizing xanthate solids and aqueous solutions arc given. Vibrational analyses are reported for sodium ethyl xanthate, trans and gauche forms, and the methyl and isopropyl analogs using a Cartesian coordinate force field derived from ab initio molecular orbital calculations.

INTRODUCTION The i.r. spectra of alkyl xanthate salts and metal complexes have been previously studied Cl-131 and some Raman spectra have been reported [ll, 121. Most vibrational assignments were made empirically; normal coordinate calculations were done in some cases. Xanthates which have been treated include: the Ni, Pd and Pt complexes of methyl xanthate [ 10, 121 and its ion [ll]; and the Ni complex of ethyl xanthate (trans form) [9, lo] and its ion [13]. Force constants from related molecules were adjusted to fit the data and the vibrational assignments were reported as potential energy distributions. As all authors have commented, there is much interaction among the CO and CS stretching modes of interest. Many authors C4-6, 10-131 have indicated that a band in the 129&1170 cm- 1 region, usually assigned to C-O-C out-of-phase stretching, is sensitive to the nature of the metal in the complex. The wavenumber increases as the metal-sulfur bond becomes stronger. The X-ray crystal structure for potassium ethyl xanthate has been reported [ 143. New Raman data reported here indicate the presence of rotational isomers in ethyl and other xanthates, and show some bands useful for xanthate characterization. A new study is reported using molecular orbital derived force fields for normal coordinate calculations.

xanthate geometry. The xanthate geometry parameters used were: O-C(S) 1.387& O-C(H) 1.436& C-S, 1.68& C-S, 1.68& C-H, (trans to C) 1.092& C-H, (gauclie to C) 1.093 A, S,C-0 122.0”, S,-C-O 113.2”, C-O--C 116.6”; the H-C-O-C dihedral angles were 60, 180 and 300”. For ethyl and isopropyl salts, additional parameters used were C-C 1.54 A and H-C-H 109.47”. In isopropyl salts the minimum energy C-O-C-C and C-O-C-C’ dihedral angles (from MO) were 80 and 200”. The crystal structure for potassium ethyl xanthate (trans form) shows a C-O-C-C dihedral angle of 180”. In this study a gauche isomer was found. The C-O-C-C dihedral angle used was 80”. If this angle were 60” there would be more steric hindrance between the CH3 and one sulfur. Using the Gaussian 80 programs [ 151, a Cartesian coordinate force field was derived from the second derivatives of the MO energy changes with respect to the Cartesian coordinate changes. This force field was used in a normal coordinate calculation to derive the wavenumbers of the vibrational spectra [lS]. The forms of the vibrations are given as Cartesian displacements of the atoms [ 151. This type of representation,

-/

671

-. cm-l

-../

625

‘.;i

620 cm-l

cm-’

CALCULATIONS This study utilized the Gaussian 80 series of programs [15] with STO-3G basis sets. The four xanthates studied are the sodium salts of: methyl xanthate, truns and gauche isomers of ethyl xanthate and isopropyl xanthate. The models used are shown in Fig. 1. The Na-S distance of 2.826 A was taken from the literature [ 161 and was used for S, (cis to C) and S, (truns to C). The molecular orbital (MO) programs [15] were used to get the MO minimum energy

*Author to whom correspondence should be sent at: American Cyanamid Company, Medical Research Division, Lederle Laboratories, Pearl River, New York 10965, U.S.A.

Fig. 1. Models used for sodium alkvl xanthates: (a) is N&CXWZHZCH3 (trans), (b) is NaS&@CH(CH&: (c) is NaS,CWCH,CH, (gauche) and (d) is NaS,CC%CH,. The C%, in-phase &d&hing ‘vibraiidn is ilkstrated 6 = stretch, B = bend). In (a) and (b) only, this vibration bends the 0-c-C angle, making the wavenumbers (from Tables l-3) higher than those for (c) and (d). 317

N. B. COLTHUPand L. PORTERPOWELL

318

which has not previously been published for these molecules, has the advantage that the relative phases of the internal coordinate changes are clearly seen. Some results are shown in Fig. 2 for the sodium salts of methyl and tram ethyl xanthates. The calculated wavenumbers are reported in Tables l-3 for the four molecules seen in Fig. 1. Force constants derived from MO calculations are generally somewhat too high and the calculated wavenumbers must be multiplied by a scale factor to match the experimental wavenumbers. The scale factors vary somewhat with the type of MO calculation used and with the particular internal coordinate combinations involved. In this work they are roughly 0.8 above about 700 cm- 1 and roughly 0.9 below. The vibrational assignments given are deduced from the Cartesian displacement calculations.

*. *.

Significantly, for the four xanthate here the scale factors are nearly the type of vibration (involving the angles). This internal consistency validity of our assignments.

molecules studied same for the same same bonds and substantiates the

SPECTRA

The ethyl, isopropyl, isobutyl and n-amyl xanthate salts were prepared by reacting the appropriate alcohol with CS2 and NaOH or KOH in naphtha [17]. No methyl xanthate salt was available for this study (it tends to be unstable in water solutions). For the methyl derivative only, the literature [ 1l] spectral bands for potassium methyl xanthate were assigned. The sodium salts reported here were all hydrated (as evidenced in

-. *. /

1183 em-l

*--. /

I I75 cm-l

Fig. 2. The vibrational modes for sodium methyl xanthate (Fig. Id) and sodium ethyl @runs)xanthate (Fig. la) between 1200 and 190 cm-’ (S = stretch, C = contract, B = bend, 0 = open, + and - indicate out-of-plane motion). The wavenumbers given are experimental value-sfrom Tables 1-2.

Vibrational

analysis

of alkyl xanthates

Table 1. Methyl

(cm-‘)

Calculation* Scale factort

3712 3693 3520 1825 1782 1735 1465 1399 1303 1263 1206 710 600 529 390 292

3010 2990 2932 1447

0.824 0.808 0.793 0.834 0.827 0.783 0.873 0.967 0.902 0.864 0.993

1430 s 1183 vs 1llOvs 1088 sh w 1045 vs 944 w 620 VW 580 VW 477 m 337 w 290 m

ion

Raman+ Observed (cm - ‘)

Infrared3 Observed (cm- i)

0.811 0.810 0.833 0.793

xanthate

319

Q

CH, OP str CH, OP str A” CH, IP str CH, OP def CH, OP def A” CH, IP def C-&C OP str CH, rk, CS2 OP str CH, rk, A” CS2 OP str, CH, rk C-&C IP str CS, IP str OCS, wag A” C-&C, SXpO OP def CS, def CS, rk

3020 m 3000w 2940 s 1447m

w VW m m

Assignment

1430 w 1188m 1112m 1088 sh w 1057 m 942 m 620 vs 580 m 477 vs 337 s 292 s

*Calculation for the sodium salt. tScale factor necessary to match observation. IInfrared and Raman data for the potassium salt from R. MATTES and G. PAULEICKHOFF, Spectrochim. Acta 29A, 1339 (1973). SAssignments from the calculations in column 1. Abbreviations used in these tables include: OP (out of phase), IP (in phase), str (stretch), def (deformation), rk (rock), tw (twist), A” antisymmetric to the plane of symmetry. The intensities are indicated as: s (strong), m (medium), w (weak), v (very), sh (shoulder).

Table 2. Sodium Calculation Tram form Scale factor (cm-‘) 3729 3710 3691 3582 3524 1843 1822 1819 1736 1640 1496 1462 1382 1342 1273 1248 1094 955 763 601 521 428 339 324 194

0.800 0.797 0.796 0.809 0.813 0.792 0.792 0.793 0.797 0.812 0.842 0.804 0.836 0.829 0.817 0.808 0.793 0.850 0.868 0.980 0.856 0.923 0.914

ethyl xanthate

Calculation Gauche form (cm - ‘) Scale factor 3732 3709 3697 3572 3527 1840 1823 1764 1726 1660 1543 1460 1402 1308 1275 1245 1080 942 705 604 569 433 373 363 303

0.802

0.833

0.830 0.798 0.789 0.851 0.887 0.852

*Gauche form in parentheses. t A” antisymmetric to plane of symmetry SObserved in H,O solution.

(hydrate)

Infrared * Observed (cm-‘) 2980 mw 2958 w 2937 w 2897 w 2868 w 1460 w 1443 w 1443 w (1415 w) 1383 w 1363 w 1260 w (1285 w) 1175s 1155s 1113 s (1085 m) 1040s 1009 m 867 w (852 w) 812 w (802 w) 662 w 446~ 310w

Raman * Observed (cm-i) 2984 2963 2939 2901 2878 1459

w w w w w w

1391 w 1363 w 1271 w (1295 w) 1182~ 1121 w (1093 w) 1063 mw 1011 w 868 w 813 w 671 s (625 s) 589 w 454 m (492 w) 404m 311 w (19l)vw$

applies to

AssignmentP CH, OP str A” CH, OP str CH, OP str A” CH, IP str CH, IP str CH, OP def, CH, def CH, OP def A” CH, OP def, CH, def CH, IP def, CH, wag CH, IP def, CH, wag CH, tw, CH, rk CC&C OP str CH, rk, CS, OP str CH, rk, CH, rk tw A” CS, OP str CCOC str, str cntr CCOC str, str str CH, rk, CH, rk A” CS, IP str OCS, wag A” COC def OCC def CS, def CH, torsion CS, rk

tram form only.

the spectra) but the other xanthates were anhydrous. Aqueous solutions of sodium or potassium alkyl xanthates can be reacted with soluble metal salts to make heavier metal alkyl xanthates [17]. The lead and

zinc xanthate complexes were prepared in this manner. The i.r. spectra of the compounds studied here compare favorably with literature spectra [3]. The i.r. and Raman results are reported in Tables

320

N. B. COLTHUP and L. PORTERPOWELL

Table 3. Sodium isopropyl xanthate (hydrate)

(cm-‘) 3730 3729 3711 3706 3635 3527 3525 1843 1830 1825 1822 1740 1728 1643 1599 1460 1436 1399 1340 1289 1168 1159 1120 1004 765 605 551 527 438 386 375 329 320 211

Calculation Scale factor

Infrared Observed (cm-‘)

0.799 0.800 0.790 0.792 0.798 0.816 0.816 0.794 0.799 0.791 0.793 0.796 0.795 0.815 0.826 0.815 0.815 0.815 0.813 0.807 0.854 0.800 0.811 0.809 0.875 0.955 0.849 0.839 0.904 0.938

Raman Observed (cm-i)

2982 mw 2982 mw 2935 w 2935 w 2900 VW 2878 w 2878 w 1463 w 1463 w 1444w 1444w 1385 m 1373 m 1339 w 1321 w 1190s 1170ms 1140ms 1090s 1040s 998 w 928 VW 907 w 812~

2984 mw 2984 mw 2942 mw 2942 mw 2881 VW 2881 VW 1456 w 1456 w 1350 w 1329 w 1192 mw 1179mw 1107 w 1049 w 912 w 820 w 669 vs

578 w 468s 442 VW 396 m 365 w

397 w 362 w

0.906 0.905

290w

297 w (191) w*

Assignment CH, OP str CH, OP str CH, OP str CH, OP str CH str CH, IP str CH, IP str CH, OP def CH, OP def CH, OP def CH, OP def CH, IP def, (CH,, CH,) IP CH, IP def, (CH,, CH,) OP CH wag, 11to O-C(H) CH wag, I to O-C(H) C-@C OP str CH, IP rk I to @C(H) CH, rk, (S)C-O str C-C-C OP str CS, OP str C-O-C IP str CH, OP rk I(to O-C(H) CH, OP rk I to C&C(H) OCC, IP str CS, IP str OCS, wag COC, CCC IP bend OCC, OCC’, CCC IP bend CCC, SCS OP bend OCC, OCC’ OP bend CH, IP torsion CH, OP torsion SCS, CCC IP bend CS, rk

*Observed in H,O solution.

l-4. Infrared spectra were run on a Perkin-Elmer 580B infrared spectrophotometer. The solid phase spectra were run as mulls in mineral oil and halogenated oil, and the aqueous solutions (in Table 4) were run between ZnS plates.

Raman data were acquired using a Spex 1403 spectrometer with a EM1 6256 cooled photomultiplier. The Spectra Physics 164 argon ion laser was operated at 488 nm with 200-400 mW power. The spectrometer bandpass was 5 cm- ’ and the integration time was

Table 4. Characteristic bands for xanthate salts

Molecule CH,aS,K, solid* C,H,aS2K, solid C,H,-O-CS,K, in H,O C,H,-O-CS,Na, solid C,H,aS,Na, in H,O i-C,H,XXCS,Na, solid i-CsH,aS,Na, in H,O i-C,Hs-O-CS,Na, solid i-C,H,aSrNa, in H,O a-CsH, ,+-CS,K, solid n-CsH, ,@-CS,K, in H,O (C,H,-O-CS,),Pb, solid (i-C,H,-O-CS,),Zn, solid

CS, in-phase stretch (COCC trams) Raman (cm-‘)

CS, in-phase stretch (COCH trans) Raman (cm-‘) 620 vs

675 vs 669 vs 671 vs 669 vs 669 vs 669 vs 676 vs 671 vs 669 vs 673 vs 659 vs 650 vs

623 m 625 vs 623 m

OCS, out-ofplane wag Raman (cm-i)

COC deformation Raman (cm-‘)

cs, out-ofphase stretch Infrared (cm-‘)

580 m 588 VW

477 vs 457 ms 456 ms 454 ms 456 ms 470 ms 472 s 446 ms 447 ms 453 mw 459 mw 447 ms 458 ms

1045 vs 1052 s 1045 s 1040s 1045 s 1040s 1038 s 1064 vs 1062 vs 1075 vs 1057 vs 1021 s 1030 s

589 VW 592 VW 590 VW 592 VW

625 m 587 VW 626 m 569 w

*Data from R. MATTESand G. PAULEICKHOFF, Spectrochim. Acta 29A, 1339 (1973).

Vibrational analysis of alkyl xanthates 0.3-1.3s/cm-‘.

The solid samples were run in melting point capillary tubes; the aqueous solutions (10 % by weight; in Table 4) were run in fluorescence cuvettes. Samples were illuminated at a 90” angle to the monochromator optic axis.

RESULTSANDDlSCUSSlON

The vibrations of methyl xanthate ion have been previously studied and the results applied to potassium methyl xanthate [ll]. Data for the methyl xanthate ion are shown in Table 1. The results of the sodium methyl xanthate MO calculations are reported in the table, and are used to assign the literature [l l] spectral bands for the closely related potassium salt. The potassium salt could not be economically calculated. The methyl xanthate free ion was also calculated, but there were only small wavenumber differences. (The differences were somewhat larger for low frequency vibrations in which the Na-S distance is changed. However, the forms of the vibrations were almost the same.) The scale factors shown in Table 1 relate the calculated and observed values and are given for convenience. The assignments involving CO and CS stretching shown in Table 1 are a little different from those given for the ion [ 1 l] and are much closer to those given for the heavier metal complexes [lo, 121. The vibrations involving CS2 deformation and rocking obviously also involve some Na-S stretching as seen in Fig. 2. Vibrations which are lower than 200 cm 1and involve motion of the metal were left out. In the crystal, more than one xanthate is associated with each metal and vice versa [ 141, so the model used in Fig. 1 is inadequate for the metal force field. In Table 2 are shown the solid state sodium ethyl xanthate (hydrate) data for both the tram and gauche forms seen in Fig. 1. The calculated frequencies are nearly the same for many vibrations but a few are different. All differences in the observed frequencies between the two isomers are indicated by reporting the gauche sodium ethyl xanthate (hydrate) data in parentheses. The scale factors given for the gauche isomer in the fourth column of Table 2 are nearly the same as those in the second column for the same vibration in the tram form. The potassium and sodium salts and the gauche isomer have not been previously analyzed, but the calculated vibrations for the nickel complex 19, lo] and ion [ 131 of tram ethyl xanthate have been reported. As expected, the sodium salt results are somewhat different from the previous work, especially in the vibrations involving metal-sulfur stretching. Table 3 shows the results for solid state sodium isopropyl xanthate (hydrate), which has not been previously analyzed. In the model used, depicted in Fig. 1, the two CO-C-C dihedral angles are 80 and 200”. Characteristic bands for xanthate salts and complexes are reported in Table 4. The i.r. band intensities are strong in the 1200-1000 cm-’ region and the

321

comparable Raman bands are weak. The reverse is true for the bands in the 7001100 cm- ’ region, except for the 0CS2 out-of-plane wag band which is weak in both i.r. and Raman spectra. The strongest Raman bands for these xanthates occur in the 676610 cm- ’ region. The 676650 cm- ’ band (Table 4) is assigned to the inphase CS2 stretching vibration in which the CO-C-C dihedral angle is not far from 180” (the trans configuration) (see Fig. la and b). In the sodium isopropyl xanthate there are two CO-C-C dihedral angles of 80 and 200”. Since one of these dihedral angles is close to 180”, this is included in Table 4 with the tram isomers. Calculations were also performed for the symmetrical isopropyl isomer where the two CO-C-C dihedral angles were 120 and 240’. It had an energy about 1 kcal/mol higher than the more stable form and the calculated CS2 in-phase stretching frequency was only 5 cm- ‘lower. The aqueous solutions of all the sodium and potassium xanthates (except isopropyl) reported in Table 4 have a second strong Raman band at 626620 cm- ’ which is roughly half as strong as the 676650 cm ’ band. This second band is assigned to the in-phase CS2 stretching vibration where one CO-C-H dihedral angle is not far from 180” (gauche for non-methyl xanthates) (see Fig. lc and d). In the methyl case this angle is 180’ but in the other xanthates (except isopropyl) this angle is about 200”. In the isopropyl case steric hindrance prevents this angle from being anywhere near 180”. The solid sodium ethyl xanthate (hydrate) has a 625 cm- ’ Raman band about equal in intensity to the 67 1 cm ’ band (Table 4) indicating roughly equal amounts of trans and gauche isomers. For solid potassium ethyl xanthate no 625 cm 1 Raman band was observed, indicating no gauche isomer, in agreement with the known crystal structure [14]. When either salt was dissolved in water the gauche isomer was observed as described above. This difference in isomer abundances for the sodium and potassium ethyl xanthate salts can be explained as a difference in the crystal structures, since the potassium ion has a larger ionic radius than the sodium ion and since the sodium salt is hydrated. The frequency shift for these rotational isomers is explained as follows. As seen in Fig. 1, these vibrations all involve in-phase CS2 stretching along with a little in-phase CO stretching and a little C-0-C bending. Consider the atom which is tram (or nearly tram) to the CS2 carbon. When this atom is hydrogen (Fig. lc and d) then this vibration does not bend the O-C-H or OPCX angles appreciably. But when this atom is carbon (Fig. la and b) then this vibration sharply bends the O-CC angle, thereby increasing the restoring force on the displaced oxygen and raising the frequency. This rotational isomer shift is exactly analogous to those in monohalogenated alkanes [ 181 with the same explanation for the shift 1193. The assignments shown in Table 4 agree with those given for methyl xanthate complexes by MATTES and PAULEICKHOFF [12]. These authors noted that, for methyl xanthate complexes with increasing metal-

N. B. COLTHIJP and L. PORTERPOWELL

322

sulfur bond strengths, the CS2 in-phase stretching frequencies decrease down to 610 cm-‘. The same type of shift occurs for the OCSz out-of-plane wagging vibration [12]. The CS* out-of-phase stretching vibrational bands are intense in the i.r. and their assignment to the region near 1050 cm- ’ is in agreement with most authors [l-13]. REFERENCES R. FELUMB,Comptes Rendus 244, 2038 (1957).

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