3920
GARYE. MACIELAND DAVIDA. BEATTY
Carbon-13 Magnetic Resonance Study of Alkyl Cyanides, Isocyanides, Isocyanates, and Isothiocyanates
by Gary E. Maciel and David A. Beatty Department of Chemistry, Universby of Cdgornka, Davia, California (Received June 7, 1966)
The C13 chemical shifts of the simple alkyl cyanides, isocyanates, and isothiocyanates as well as the cyanate and thiocyanate ions were found to cover the range of -5.7 to 10.8 p.p.m. (with respect to benzene), while the Cla shifts of the simple alkyl isocyanides and cyanide ion range from -28 to -40 p.p.m. These shifts seem to reflect familiar similarities between the valence-bond descriptions of these compounds and those of carbon dioxide (3.7 p.p.m.) or carbon monoxide (-53.4 p.p.m.). In the methyl compounds the methyl C13 shifts vary monotonically but not linearly with the shifts of the other carbon in the molecule, covering the range from 99 to 128 p.p.m.
Introduction The molecular structures of simple alkyl cyanides, isocyanides, isocyanates, and isothiocyanates have served as interesting subjects of investigation by a variety of physical techniques. Infrared1-” investigations and microwave studies12-18 have been carried out on the methyl compounds in these classes, and useful structural information has been derived. I n addition, R a m a ~ ~ l -and/or ~ t ~ electronic2 spectra have been determined for some of these compounds. Pauling, Wheland, and other^^^^^^^ l9 - 2 2 have interpreted the data from these and other physical measurements in terms of the valence-bond structures assigned to the compounds, and various relationships between their proposed structures and between these and the structures of carbon monoxide and carbon dioxide have been formulated. Recently, infrared evidence indicating hydrogen bonding of hydroxyl hydrogens to the carbon atoms of isocyanide groups has been p r e ~ e n t e d ,and ~ ~ ,the ~ ~ application of organic cyanides and isocyanides as ligands in transition metal complexes has recently been established. 25-27 In the light of the considerable qualitative success with which carbon-13 chemical shifts have been correlated to electronic structural properties in some classes of compounds such as substituted benzeneslZ8 carbonyl ~ompounds,29-~2 vinyl and substituted ethanes and me thane^,^^ we have determined the CI3 magnetic resonance spectra of methyl, ethyl, The Journal of Physical Chemistry
and cyclohexyl compounds of the alkyl cyanide, is0 cyanide, isocyanate, and isothiocyanate systems, as well as their related anions. Some interesting trends
(1) F. A. Miller and W. B. White, 2.Elektrochem., 64, 7017 (1960). (2) C. N. Ramachandra Rao, J. Ramachandran, and S. Somasekhara, Current Sci. (India), 27, 474 (1958). (3) A. J. Castoulas and R. L. Werner, Australian. J . Chem., 12, 601 (1959). (4) R. L. Williams, J . Chem. Phys., 25, 656 (1956). (5) D. J. David, Anal. Chem., 35,37 (1963). (6) G. L. Caldow and H. W. Thompson, Spectrochim. Acta, 13, 212 (1958). (7) J. P. Jesson, H. W. Thompson, ibid., 13, 217 (1958). (8) E. Lieber, C. N. R. Roa, and J. Ramachandran, ibid., 13, 296 (1959). (9) N. S. Ham and J. B. Willis, ibid., 16, 279 (1960). (10) H . W. Thompson and R. L. Williams, Trans. Faraday SOC.,48, 502 (1952). (11) M. G. Krishna Pillai and F. F. Cleveland, J . Mol. Spectry., 5, 212 (1960). (12) S. N. Ghosh, R. Tambarulo, and W. Gordy, J . Chem. Phys., 21, 308 (1953). (13) C. C. Costain, ibid., 29, 864 (1958). (14) R. F. Curl, Jr., V. M. Roa, K. V. N. Sastry, and J. 9.Hodgeson, ibid., 39, 3335 (1963). (15) H. H. Nielsen, Phys. Rev., 77, 130 (1950). (16) M. Kessler, H. Ring, R. Tambarulo, and W. Gordy, ibid., 79, 54 (1950). (17) C. I. Beard and B. P. Dailey, J . Am. Chem. SOC.,71,929 (1949). (18) T . S. Jaseja, Proc. Indian Acad. Sci., SOA, 108 (1959). (19) L. Pauling, “The Nature of the Chemical Bond,” Cornel1 University Press, Ithaca, N. Y., 3rd Ed., 1960, pp. 265-274, 331-338.
C13 CHEMICAL SHIFTSOF ALKYLCYANIDES
3921
in chemical shift emerge which corroborate the ideas previously stated regarding analogies in electronic structures.1 9 3
Results The principal peaks of interest in the C13 spectra of the compounds of this study were those due to the resonance of the carbon atom of the cyanide, isocyanide, Experimental Section isocyanate, or isothiocyanate groups. These peaks were found in approximately the same region of the Materials. Cyclohexyl cyanide was prepared from spectrum as those due to the resonance of aromatic cyc'lohexanecarboxylic acid via the acid chloride and carbons, and covered a range from -39.9 p.p.m. with amide. The final step was accomplished by refluxing respect to benzene for the cyanide ion to 10.8 p.p.m. the amide in toluene with a twofold excess of phosphorus for acetonitrile. Peaks due t o the alkyl groups were oxychloride for 1 hr. The organic mixture was dealso observed and in the case of methyl compounds the canted from solids and distilled, affording a colorless simple multiplet structure and relatively favorable liquid, b.p. 80-81' (19 mm.), n 2 41.4502 ~ [lit.E5,B6 b.p. signal-to-noise ratios permitted reliable determinations b.p. 69-70 (12 mm.), ~ 75-76 (16 mm.), n l 9 1.45W5; of the corresponding shifts. These were found to 7225D 1.449736]. range from 99.2 p.p.m. for methyl isocyanide to 128.2 Methyl isocyanide was prepared by an adaptation p.p.m. for acetonitrile. These chemical shift data are of the method reported by Jackson and M c K ~ s i c k ~ ~collected in Table I and in Table 11, which includes for the preparation of ethyl isocyanide, involving the some relevant data from other source^.^^-^* reaction of methyl iodide with silver cyanide, and the The only shifts in Table I which have appeared treatment of the resulting complex with aqueous previously in the literature are those of acetonitrile, potassium cyanide. Distillation afforded a nearly and there appear to be larger discrepancies than one colorless material, b.p. 56-59' [lit.459-60'], which was would anticipate from expected experimental error of found by gas chromatography to be contaminated by about 15% methyl iodide. (20) G. W. Wheland, "Resonance in Organic Chemistry," John Ethyl isocyanide was prepared by the method of Wiley and Sons, Inc., New York, N. Y., 1955, pp. 111-115, 156, Jackson and M c K u ~ i c k affording ,~~ a colorless liquid, 180-182. b.p. 77-79' [lit.37b.p. 77-79']. (21)J. W.Linnett, A'ature, 199, 168 (1963). The following reagents were used as obtained from (22)R. G. Gillis, J . Org, Chem., 27, 4103 (1962). (23)L. L.Ferstandig, J . Am. Chem. Soc., 84, 1322 (1962). their suppliers : cyclohexyl isocyanide and cyclohexyl (24)A. Allerhand and P. von R. Schleyer, ibid., 85, 866 (1963). isothiocyanate were obtained from Aldrich Chemical (25)L. Malatesta, Progr. Inorg. Chem., 1, 283 (1959). Co., cyclohexyl isocyanate from K and K Laboratories; (26) T . L. Brown and M. Kubota, J . Am. Chem. SOC.,83, 331 ethyl isocyanate was an Eastman Practical chemical (1961). (redistilled, b.p. 60-61 ') ; ethyl isothiocyanate, methyl (27) T . L. Brown and M.Kubota, ibid., 83, 4175 (1961). isothiocyanate, methyl isocyanate, and propionitrile (28) G. E. Maciel and J. J. Natterstad, J . Chem. Phys., 42, 2427 (1965),and reference therein. were Eastman White Label samples; and the aceto(29) G. E. Maciel and J. J. Natterstad, ibid., 42, 2752 (1965),and nitrile, potassium cyanide, potassium cyanate, and references therein. potassium thiocyanate were J. T. Baker reagents. (30)D. H. Marr and J. B. Stothers, Can. J . Chem., 43, 596 (1965), N.m.r. Spectra. The carbon-13 magnetic resonance and references therin. (31)J. B. Stothers and P. C. Lauterbur, ibid., 42, 1563 (1964). spectra were obtained a t a fixed frequency of 15.1 Mc./ (32)G. B. Savitsky, K. Namikawa, and G. S. Zweifel, J . Phys. sec, by measuring natural abundance C13 resonance, Chem., 69, 3105 (1965). using dispersion mode and rapid passage conditions as (33)G. E. Maciel, ibid., 69, 1947 (1965). described by L a u t e r b ~ r . Details ~~ of the procedure (34) H. Spiesecke and W. G. Schneider, J . Chem. Phys., 35, 722 are reported elsewhere.39 The results may be con(1961). (35) R.Paul and S. Tchelitcheff, Bull. soc. chim. France, 470 (1949). sidered reliable to about *0.8 p.p.m. All chemical (36) S. M. McElvain and R. E. Starn, Jr., J . Am. Chem. Soc., 77, shifts were determined on neat liquids except those of 4571 (1955). potassium cyanide, potassium cyanate, and potassium (37) H.L. Jackson and B. C. McKusick, Org. Syn., 35, 62 (1955). thiocyanate which were obtained as saturated aqueous (38)P. C.Lauterbur, J . Am. Chem. Sac., 83, 1838 (1961). solutions. I n order to estimate the possible effect of (39) C.P.Nash and G. E. Maciel, J . Phys. Chem., 68,832 (1964). the methyl iodide contaminant on the methyl isocyanide (40)R. Ettinger, P.Blume, A. Patterson, Jr., and P. C. Lauterbur, J. Chem. Phys., 33, 1597 (1960). spectrum, we obtained the chemical shifts of aceto(41) D. D. Traficante and G. Maciel, J . Phys. Chem., 69, 1348 nitrile both neat and in a l : l mixture with methyl (1965). iodide. Within the limits of experimental error no (42) R.A. Friedel and H. L. Retcofsky, J . Am. Chem. Soc., 85, 1300 medium effect was detected. (1963). Volume 69, Number 11 November 1966
3922
GARYE. MACIEL AND DAVID A. BEATTY
sented by the analogous resonance hybrids of I, 11, and 111,where R represents an alkyl group.
Table I : C13 Chemical Shifts of Alkyl Cyanides, Isocyanides, Isocyanates, and Isothiocyanates, P.p.m. with Respect t o Benzene
-
+
:czo:
+
t--,
Alkyl group
SNC
6CN
7.1 Cyclohexyl 7.7 Ethyl Methyl ( d c ~ ~ ) ‘ 10.8(128.2)
:c=o: * :c=o:
6NCS
SNCO
-28.0 -27.7 -30.0(99.2)
..
:C-O:-t--,
5.0
6.1 7.2(102.4)
-3.7 -2.1 0.0(99.4)
The numbers in parentheses represent &a8.
Ia
-
+
Ib
IC
: C ~ p u ’++ : :C-N:2IIa
Id
..
t+
IIb
Table I1 : Some Relevant C13 Chemical Shifts, P.p.m. with Respect to Benzene
coz
co
CSP
3.7“
-53.4”
- 6 5 , Ob’’
(NCO) -
(NCS) -
n-Ca HOC1=c1a-CH3
CHsC=C-C”Ha
- 1.1”J
-5.7”’f
48.7 d
125“
(CN) -
-39.9””
D a t a from ref. 40. Data from ref. 43. Present work. Data from ref. 41, average shifts of a and 6 carbons. Data Data obtained on saturated aqueous solutions from ref. 42. of the potassium salts.
the dispersion mode, rapid passage method. 34*43,44 Frei and B e r n ~ t e i ndetermined ~~ the chemical shifts for acetonitrile on a sample which was doubly labeled with carbon-13, using the more precise absorption mode technique. Their results, 10.4 and 127.8 p.p.m. with respect to benzene for the cyanide and methyl carbons, respectively, agree with our values reported in Table I within our experimental error.
Discussion Examination of the shifts in Tables I and I1 reveals that exclusive of carbon disulfide and acetylenic and methyl carbons which are considered in later sections, the carbon groups studied in this investigation can be separated into two classes, within which electronic similarities might be expected: (a) the group consisting of carbon monoxide, cyanide ion, and alkyl isocyanides with chemical shifts in the range -28 to -53 p.p.m., and (b) the group consisting of carbon dioxide, alkyl cyanides, isocyanates , and isothiocyanates, and the cyanate and thiocyanate ions with chemical shifts in the range -10 to 11 p.p.m. These groups are discussed separately below. Species qf the CO Type. Pauling,lg Wheland,20 and others25have pointed out the similarities in formulas and chemical properties among the “terminal carbon” species carbon monoxide, cyanide ion, and organic isocyanides. All three can be formally repreThe Journal of Physical Chemistry
-:C c E+- R ++ IIIa
f
..
:C-K-R
c--?c
IIIb
I n each case, presumably the most important contributor is an a-structure, with a formal charge of -1 and a local electron density approaching 5 on the terminal carbon. The remaining species listed in Table I and I1 cannot be represented reasonably in this manner. From the available physical data, Paulinglg has estimated that structure I a contributes 50% to the electronic state of carbon monoxide, with the equivalent structures ICand Id each contributing 20% and I b contributing 10%. Similarly, he has estimated that the contribution of IIIa to methyl isocyanide is 74% with IIIc and IIId contributing 13% each. Since the formulas derived by Karplus4j and Pople45p46for the dominant paramagnetic contribution to C13 chemical shifts show a general increase of shielding with increasing local electron density (assuming other factors such as average electronic excitation energies remain constant), one might expect the largest shielding among the series I, 11, 111, for the species with the highest contribution from the corresponding a-structure. If one is willing to place the relative importance of IIa intermediate between that of Ia and IIIa for their respective species, then the observed shifts appear consistent with Pauling’s representations. That I I a and IIIa should be more important contributors than I a to their corresponding species seems reasonable (43)P.C. Lauterbur, J . Chem. Phys., 26,217 (1957). (44)K.Frei and H. J. Bernstein, (bid., 38, 1216 (1963). (45) M. Karplus and J. A. Pople, ibid., 38, 2803 (1963). (46) J. A. Pople, Mol. Phys., 7, 301 (1964).
C13 CHEMICAL SHIFTSOF ALKYLCYANIDES
in terms of Pauling's estimates, and since the great electronegativit8yof a formally positive oxygen would seem to render Ib, IC,and Id (none of which have formally doubly charged atoms) more important than their analogs in I1 and 111. The relative importance of I I a to I1 compared to IIIa to I11 is more difficult to predict. The implication of the C13 data, that IIIa may make a greater contribution to I11 than IIa does to 11, is difficult to rationalize in terms of expected favorable charge distributions; however, a rather large contribution of IIc and IId to I1 seems consistent with the rather large contribution of analogous structures often attributed to transition metal complexes, as represented by the structure IV for the ferro-
3923
Va
Vb
VC
VI0
VIIa
VId
VIIb
VIIc
IV cyanide i0n.19 Similarly, a CN double-bond character of about 50% has been proposed to account for the spectroscopically determined molecular geometry of the octahedral complex [Fe(CNCH&I2+ between methyl isocyanide and FeII.lg I n any case, the C13 chemical shifts of species I1 and I11 differ only by about 11 p.p.m., and other factors such as average electronic excitation e n e r g i e ~ would ~ ~ , ~ have ~ to be taken into account in any complete explanation. Solvent effects may also play a role. Species of the COz T y p e . I n this section we consider the C13 chemical shifts of species of the type XCY, in which the central carbon atom bears zero formal charge in the most important contributor. Presumably, the central carbon atom is sp hybridized in all these species and the three central atoms are collinear in each case. Wheland20 has expressed the opinion that organic isocyanates are structurally somewhat analogous to carbon dioxide. Also, the cyanate and thiocyanate ions are isoelectronic with carbon dioxide, and alkyl cyanides bear some resemblance in electronic structure. These similarities can be summarized in the corresponding resonance hybrid formulations V, 'I1' -and v111*'g'20 Theisocyanates O r isothiocyanates
VIIIa
Vd
VIId
VIIIb H&--C=N VIIIC
:
can be thought of in terms of the formula VI or VI1 if one considers replacing an appropriately situated electron pair on nitrogen by a covalent bond to an alkyl group, with the consequent increase of the formal charge on nitrogen by unity; of course, the resulting alkyl analog of VIc or VIIc would be especially unstable because of the nonlinearity of the OCNC or SCNC s y ~ t e m . ' ~ ~ ~ ~ P a ~ l i n g has ' ~ concluded from physical measurements on carbon dioxide that the normal state of the molecule can be described as arising from about a 25% contribution of each of the structures indicated for V, and that the experimentally determined dimensions of methyl isothiocyanate are compatible with what one would expect for resonance among the three most stable structures VIId, VIIc, and VIIa or VIIb (depending on the orientation of the methyl group). For the latter compound, a controversy has existed concerning the interpretation of vibrational spectra with regard to the importance of the structure corresponding to VIIc. 3~8,9,47 For isocyanic acid, Pauling concluded (47) J. H. H i b b n , "The Raman Effect and its Chemical Applications,'' Reinhold Publishing Gorp., New York, N. Y., 1939, p. 283.
Volume 60, Number I1
November 1066
3924
GARYE. MACIELAND DAVIDA. BEATTY
that the physical properties could be explained by It is also worth noting that a considerable difference assuming approximately equal contributions from the exists between the CI3 chemical shifts of similarly structures VId and VIb or VIa, with VIc making a substituted nitrile and acetylenic carbons for which slightly smaller contribution because the H-N-C resonance structures formally analogous to VI11 can bond angles is only 128’.19 For methyl cyanide, he be written. Perhaps this difference, about 38 p.p.m. estimated that structures of the type VIIIa and VIIIb for the methyl-substituted case, can be rationalized contribute a total of about 17%, the remaining conmore satisfactorily by attributing it to the different tribution made by structures VIIIc. Thus all of these polarities of the corresponding triple bonds, as represpecies can be represented as resonance hybrids besented in the structures IX. One would expect a tween structures which have central carbons with a formal charge of zero and a formal electron density of four. I n view of the preceding discussion of the similarities in electronic structure between alkyl cyanides, isocyanates, isothiocyanates, carbon dioxide, and the greater contribution of the energetically equivalent cyanate and thiocyanate ions, it is not surprising to structures I X b and IXc and a correspondingly lower find the C13 chemical shifts of these species covering chemical shift (larger magnitude for up) when Y: a range of only about 16 p.p.m. Again, a satisfactory is the more electronegative N: rather than the less explanation of these small differences would require electronegative C (CH2),CH3. consideration of polar effects and electronic excitation Alkyl Groups. Table I displays a monotonic relae n e r g i e ~ ,as ~ ~well , ~ ~as medium effects. One perplextionship between the methyl C13 chemical shifts and ing feature of the data of Tables I and I1 is the exthe shifts of the corresponding carbon atoms in the tremely low chemical shift of carbon disulfide compared groups to which they are attached. While the relato the shifts of the other species under discussion in this tionship is by no means linear, it may indicate a presection. Thus, the shift of carbon disulfide is seen dominant influence of magnetic anisotropy in these to be nearly 70 p.p.m. lower than that of carbon dilinear or pseudo-linear systems. Thus, in a case of oxide. I n view of their similar electronic structures, true axial electronic symmetry, as with isocyanide this might not have been anticipated, especially since and cyanide substituents, the resulting increase in the expected difference in bhe bond polarities between chemical shift due to the zero value of up,, would be these two compounds would seem t o predict a chemical manifested in the same sense all along the molecular shift difference of the opposite sign. Most likely, the axis.54 While the electronic distributions in the explanation of the observed shift difference is due t o a isocyanate and isothiocyanate groups are not axially much smaller average electronic excitation energy A E symmetrical, the still-present group magnetic anisotfor carbon disulfide. The factor (AE)-l occurs in each ropy may give similar effects at both carbon nuclei, term of the expression derived by Karplus and Pople if the methyl groups do not lie too far from especially for AuP, the local paramagnetic term for the electronic NCO or NCS axis. Of course the inductive and the shielding of a carbon-13 n u c l e ~ s . Since ~ ~ ~ this ~ ~ term resonance influences of these groups on the electron is expected to dominate the shifts, a smaller value of density of the methyl carbon may also be important. hE (which leads to a larger magnitude for Au,), gives rise to a lower shielding or chemical shift value. While it is usually difficult to estimate a priori a reasonable value for AE, the evidence seems qualitatively satis(48) S. F. Mason, Quart. Rev. (London), 15, 287 (1961). factory for a comparison of AE for carbon disulfide (49) E. C. Y.Inn, K. Watanabe, and M. Zelikoff, J . Chem. Phys., and carbon dioxide. The latter compound is found 21, 1648 (1953). to have electronic absorption only oin the far-ultra(50) J. W.Sidman, J . Phys. Chem., 61, 253 (1957). (51) L. N.Liebermann, Phys. Rm.,59, 106 (1941). violet region4*f49(1475, 1332, 1121 A., etc.), whereas (52) L. N. Liebermann, ibid., 60, 496 (1941). carbon disulfide is found to exhibit electronic absorp(53) V. K.Wieldand, Helv. Phys. Acta, 31, 555 (1958). tions in the near-ultraviolet regionm-j3 (3500,3300 i.). (54)J. A. Pople, W. G. Schneider, and H. J. Bernstein, “HighThus the much lower shift of carbon disulfide can be Resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Go., rationalized. Inc., New York, N. Y., 1959,pp. 178-180.
The Journal of Physical Chemistry
