Vibrational bands of isothiocyanates, thiocyanates and isocyanates

SpectrochilnicsActa. 1958, Vol. 13, pp. 212 to 216. PergamonPress Ltd., London. Printed in Northern Ireland

Vibrational bands of isothiocyanates, thiocyanates and isocyanates G.

L.

CALDOW

and H. W.

THOMPSON

The Physical Chemistry Laboratory, Oxford (Received 10 July 1968) Ah&act-Vibrational absorption bands of some isothiocy&nates, thiocyanates and isocyanates in the region 2000-2300 cm-l have been measured,using dilutesolutions in carbontetrachloride or chloroform. In many cases the bands are not single, but no explanation can be given for their complexity. The variation of the main vibrational frequency and band intensity of the chromophoric group with different aromatic ring substituents has been examined. With isothiocyanates the vibration frequencies are higher in chloroform than in carbon tetrachloride, but with thiocyanates there is no such solvent effect. RECENT papers from this laboratory have reported data on the effect of substituents in an aromatic ring upon the vibration frequency and band intensity of a chromophoric group attached to the ring. Interesting empirical relationships have been discovered with nitriles [l], anilines [2], phenols [3] and other compounds [4] and an interpretation has recently been suggested in terms of the electronic effects of the substituent groups [5]. This paper describes results with aryl isothiocyanates, thiocyanates and isocyanates, each of which has strong absorption between 2000 and 2200 cm-l. Alkyl isothiocyanates have also been measured for comparison. Little has been reported previously about the characteristic frequencies of these compounds. A Perkin-Elmer 21 spectrometer with rock-salt prism was used, the effective slit width being about 8 cm- i. In estimating the intensities the bands were recorded for solutions of different concentration and path length, and extrapolation was carried out by the usual Wilson-Wells method. In the tables below, the intensity A is given in absolute units, molecules, cm, c/s, log,. The solutes were either special preparations or commercial products, repurified before use. The aryl isothiocyanates have an intense band in the range 2000-2200 cm-l, but there are other bands or shoulders, and the main band is very broad ( Avllza N 100 cm-l). Some examples are shown in Fig. 1, and the results are given in Table 1, the value of A referring to the total absorption in this region. The significance of the large band width, and of the multiple bands is not yet clear. It may be noted that the frequencies are higher in chloroform than in carbon tetrachloride, which is the converse of the results with many other solutes. Change of solvent has little effect upon the observed band intensity. The results for aryl thiocyanates are given in Table 2. The apparent half band widths are about 15 cm-l and the intensities are much lower than those of the isothiocyanates, and of about the same magnitude as those for the C=N group in nitriles. The vibration frequencies are not significantly altered on passing from carbon tetrachloride to chloroform, as with the substituted benzonitriles [6].

212

Vibrational bands of isothiocyanates, thiocyamk%

I-

and isocyanates

v v

(4)

(1)

2300

2100

2300

1900

2100

1900

cm”

Fig. 1. Phenyl

isothiocyanates. (I), (2), (3): Phenyl, p-ethoxyphenyl and p-chloro in carbon tetrachloride. (4), (5), (6): As above in chloroform.

Table

1. Phenyl

isothiocyanates

-

-

Carbon tetrachloride f3 Y

(cm-l)

/

(loeif)

p-C,H,O 2.5 diC,H,O 3COOH-40H p-Cl m-COOH p-NO, 2.3 (CH),

-0.268 -0.25 -0.20 -0.002 0 +0.227 +0,355 + 0,78

2057 2080 2040

7.4 7.0 8.1

2065 2052

7.8 8.9

2085

-/ 213

B

(cm-l)

-

-

P-C%0

Chloroform _---

_I__-

/

(106 A) ---i-

2130 2120 2050 2132 2112 2112 2115 2045

8.4

- -___I

I

7.5 7.9 7.9 7.6 7.7 8.8 8.4 10.4

G. L. CALDOWand H. W. THOMPSOX Table 2. Phenyl thiocyanates

u

/

Substituent

Carbon tet;accde

__

Chloroform Y (cm-l)

____~ -0.73 -0.66 -0.357 -0.015 + 0.05 +0.245 +0.355 +0.778 + 1.578

p-N(CH,), p-NH, p-OH p-CH,CO.NH 3N0,.4NH, p-COOH m-COOH

p-so, 24 diN0,

(10’ A)

, 2164 2166 2168 2167

2161 2161 2164 2168 2167 2169 2170 2175 2174

2174 2173

2.6 2.4 2.0 2.0 1.8

1.4

Table 3. Phenyl isocyanates Carbon tetrachloride Substituent

-

u v (cm-l)

-0.17 -0.17 -0.069 0 + 0*009 +0*17 1-0.227 + 0.232 +0.71 +0.77s +o.s

P-C% O-CH, m-CH, -

---

P-C,% 3.4(CH), p-Cl p-Br Y&NO, p-50, o-so,

2280 2282 2273 2267 2272 2273 2275 2269 2272 2269 2271

_

o-CHaO 2.3(CH),

-!

(106 -9) 7.1 7.3 7.4 7.0 S.4 84 7.1 54

7.7 8..5 7.8 ---__ 7.8 7.8

224312257 2277

Table 4. Alkyl isothiocganates,

I

Carbon tetrachloride

R.NCS Chloroform

I

R

I

v (cm-l)

I

(106 A)

v (cm-l)

2.0 5.9 5.2 3.6 6.0 9.7

2137 2130 2113 2110 2094 2000 1998

(106 A) __-

I 2113 2103 2100 2085 2083 1990

CH,

C,C

CH,=CHCH, n-‘&H, isoC,H, C&H&O p-CH,OC,H,CO

I

I 214

,

I

1.9 4.8 5.3 3.7 5.5 9.8 8.8

Vibrational bands of isothiocyanates, thiocyanatcs and isocyanates

v

-

2300

I

I

2100

I

v I

,

lQO(

-1

2300

I

I

2100

I

(4)

(6)

I

I

19OC

Fig. 7. Alkyl isothiocyanates. (l), (2), (3): Ethyl, isopropyl, n-propyl in carbon tetrachloride. (4), (5), (6): As above m chloroform.

The aryl isocyanates, which polymerize easily, were all redistilled under reduced pressure just before use. The intense absorption occurs in the range 2200-2300 cm-l, and there are shoulders to the main bands, whose half width is 25-35 cm-l. Table 3 gives the results, which agree satisfactorily with earlier data of DAVISON [7] and also with measurements in this laboratory by MIDDLETON [S]. Some liquid isocyanates were studied by HOYER [9]. A number of aliphatic isothiocyanates were also measured. These have an intense absorption near 2100 cm-l, but the band splitting is more marked than with the compounds discussed above. Fig. 2 shows some typical examples, and the results are summarized in Table 4 where the intensity A again refers to the total absorption in this region. It is impossible to interpret the band splitting. In some cases rotational isomerism could occur, but this explanation could not apply to the multiple band structure found with the aromatic derivatives above. The results for the aryl compounds listed in Tables l-3 have been considered from the standpoint of the relationships found with other series. With aryl isothiocyanates and isocyanates the plots of v against CT,the Hammett constant of the subst’ituent, show scatter although with aryl thiocyanates this plot is roughly 215

G. L. CALDOWand H. W. THOMPSON:Vibrational bands of isothiocyanates thiocyanates and isocyanates

linear with a small positive slope. The plots of log A against 0 are roughly linear for isothiocyanates and thiocyanates, with small positive and negative slopes respectively. With aryl isocyanates the band intensity is essentially independent of cr. However, the occurrence of shoulders, and the broad nature of some bands complicates the problem as regards both frequency and intensity plots. On the other hand, the small slope of the log A/G plot is consistent with the value now found for the -SCN substituent [lo] if the general arguments recently suggested by KRUEGER and THOMPSON are valid [5]. Acknowledgenzents-Many of the compounds used were kindly supplied by Imperial Chemical Industries, Dyestuffs Division, to whom we express our thanks. We are also grateful to the Hydrocarbon Research Group for financial help, and to the board of electors for the award of a Shell Postgraduate Scholarship to one of us (G. L. C.).

References [l] [2] [3] [4] 151 [6] [7] [8] [9] [lo]

THOMPSONH. W. and STEEL G., Trans. Faraday Sot. 1956 52 1451. KRUEGER P. J. and THOMPSON H. W., Proc. Roy. Sot. A 1957 243 143. STONE P. J. and THOMPSON H. W., Spechochinz. Acta 1957 10 17. THOMPSON H. W., Molecular Spectroscopy Symposium. Institute of Petroleum KRUEGER P. J. and THOMPSON H. W., Proc. Roy. Sot. 1958. In press. MANDER M. R. and THOMPSON H. W., Trans. Faraday Sot. 1957 52 1451. DAVISON W. H. T., J. Chenz. Sot. 1953 3712. MIDDLETON R. D., Dissertation, Oxford 1956. HOYER H., Chem. Ber. 1956 89 2677. MCDANIEL D. H. and BROWN H. C., J.Org. Chem.

1958

23 420.

1958.