Nuclear quadrupole resonance of nitrogen-14 in some acetonitrile derivatives

JOURNAL

OF MAGNETIC

RESONANCE

l&282-286

(1975)

Nuclear QuadrupoleResonanceof Nitrogen-14 in Some Acetonitrile Derivatives SHINZABURO

ONDA,

RYUICHI

IKEDA,

MASAJI

DAIYU

NAKAMURA,

AND

KUBO

Department of Chemistry, Nagoya University, Chikusa, Nagoya 464, Japan Received November 21, 1974 The pure quadrupole resonance of cyano nitrogen in cyanoacetic acid, methyl cyanoacetate, ethyl cyanoacetate, ethylene cyanohydrin, acetone cyanohydrin, iminodiacetonitrile, methylaminoacetonitrile, and cyanoacetamide was observed at liquid nitrogen temperature. The resonance frequencies of imino nitrogen and amide nitrogen were also detected for iminodiacetonitrile and cyanoacetamide, respectively. The observed coupling constants of cyano nitrogen were interpreted in terms of the inductive effect of various substituents. The ionic character of N-H and N-C bonds was estimated from the observed resonance frequencies of imino nitrogen in iminodiacetonitrile. INTRODUCTION

We have previously studied the nitrogen-14 quadrupole resonance of cyano nitrogen in aliphatic and aromatic nitriles (1-4). Colligiani et al. also have investigated extensively the nitrogen-14 quadrupole resonance of various nitriles (5). These studies indicate that the quadrupole coupling constant of cyano nitrogen depends to a great extent on the nature of substituents R in nitriles, RCN, even when the substituents are not involved in conjugation with the cyano group. This suggests that the inductive effect of substituents alters charge distribution in cyano nitrogen to a great extent. The present investigation has been undertaken in order to clarify this point. EXPERIMENTAL

SECTION

The pure quadrupole resonance of nitrogen-14 was observed at liquid nitrogen temperature by means of a Pound-Watkins type spectrometer already described (6). Frequency modulation was used for the determination of resonance frequencies, while Zeeman modulation was employed for the assignment of resonance signals to v’ and v’r. Resonance frequencies were determined by use of a Model TR-5178 frequency counter from Takeda Riken Company. All of the compounds investigated were procured from commercial sources and purified by distillation or recrystallization from organic solvents. Since cyanoacetic acid, iminodiacetonitrile, and cyanoacetamide form solids at room temperature, they were melted in glass tubes to increase the filling factor and were allowed to solidify by slow cooling. Other compounds liquid at room temperature were frozen gradually in sample tubes and cooled with liquid nitrogen. After being melted at about 123”C, cyanoacetamide yielded transparent crystals in a sample tube. On standing, they Copyright 0 1975 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

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turned into white powder in a few days, suggesting that some sort of phase change took place. The transparent crystals showed quadrupole resonance signals, whereas no resonance could be detected with the powder sample. RESULTS

Since nitrogen-14 has a nuclear spin equal to unity, one can usually observe two resonance frequencies, v’ and v”, when the asymmetry parameter q is finite. v’ = +eQq(3 + r),

v” = $eQq(3 - y).

Ul

Here eQq denotes the quadrupole coupling constant in frequency units. Table 1 shows the observed frequencies, quadrupole coupling constants, and asymmetry parameters of various acetonitrile derivatives. Acetone cyanohydrin shows six resonance frequencies including closely spaced doublets of v’ and v” . For these lines, a one-to-one correspondence could not be made between v’ and v”. Therefore, the frequencies of the doublet lines were averaged to evaluate eQq and 9. TABLE

PURE QUADRUPOLE RESONANCE FREQUENCIES, PARAMETERS 01; 14N IN SOME ACETONITRILE

I

QUADRUPOLE DERIVATIVES Y’ (kHz)

Compound NCCH&OOH NCCH,COOCH, NCCHZCOOC,H, NCCH,CH,OH NCCOH(CH&

Cyanoacetic acid Methyl cyanoacetate Ethyl cyanoacetate Ethylene cyanohydrin Acetone cyanohydrin

(NCCH,),NH

Iminodiacetonitrile

NCCHZNHCHa NCCH#ZONHz

Methylaminoacetonitrile Cyanoacetamide

NH CN CN CN NH2

2943.4 2959.8 2998.8 2879.2 3091.1 2995.9 2745.2 4372.3 2932.3 3005.5 2843.0 2367.7

COUPLING CONSTANTS, AT LIQUID NITROGEN Y”

(kHz)

2797.1 2840.3 2863.0 2839.7 2923.1 2897.3 2621.6 3511.6 2754.2 2809.6 2713.0

AND ASYMMETRY TEMPERATURE

eQq(kHz)

\ j

q

3827.0 3866.7 3907.9 3812.6

0.076 0.062 0.070 0.021

3969(av)

O.O7(av)

3577.9 5255.9 3791 .o 3876.7 3704.0

0.069 0.32s 0.094 0.101 0.070

Colligiani et al. (5) have studied the nitrogen-14 quadrupole resonance of methyl cyanoacetate and iminodiacetonitrile, and found a pair of v’ and v” frequencies for each compound. They observed that the frequencies of iminodiacetonitrile agreed very well with those of the low-frequency pair of the present investigation. However, their data for methyl cyanoacetate do not agree, within experimental errors, with those observed by us. This suggests a possibility of polymorphism as was found by Colligiani et al. for pimelonitrile (7). DISCUSSION

Cyanoacetic acid, methyl cyanoacetate, ethyl cyanoacetate, and ethylene cyanohydrin containing only one nitrogen atom in a molecule show a pair of resonance lines, indicating that all these molecules are crystallographically equivalent in each crystal at liquid nitrogen temperature.

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Although there is a single nitrogen atom in an acetone cyanohydrin molecule, three sets of V’ and V” frequencies were observed, indicating the presence of three nonequivalent molecules in crystals. In order to check a possibility of two or three crystal modifications coexisting in the crystals, resonance frequencies were measured at various temperatures above liquid nitrogen temperature. The resonance frequency of the six absorption lines decreased monotonically with increasing temperature. The intensities of the resonance signals were approximately equal to one another and were fairly strong except for the lowest-frequency set of v’ and v” . The lowest-frequency v’ and v” lines yielding an anomalously low quadrupole coupling constant showed rather broad, weak signals at all temperatures investigated. All of the six absorptions became gradually weaker with increasing temperature and disappeared in the noise level at approximately the same temperature (about 195 K), far below the melting point (254 K) of this compound. The crystals showed a definite melting point regardless of the method of crystallization. These facts indicate that they were not a mixture of different crystal modifications. Accordingly, the unit cell of this crystal must contain at least three crystallographically nonequivalent molecules. Iminodiacetonitrile yielded two sets of v’ and v” frequencies. This indicates that all the molecules are crystallographically equivalent in crystals, because two chemically nonequivalent nitrogen atoms exist in a molecule. Previously, the present authors (3) and Colligiani et al. (5) studied the nuclear quadrupole resonance of nitrogen in various aliphatic nitriles and found that v’ and v” frequencies appear in the frequency range of 2.7-3.2 MHz. On the other hand, it has been known that nitrogen atoms of aliphatic secondary amines show v’ and v” frequencies in the range of 3.0-4.5 MHz (8-10). Accordingly, the high-frequency and low-frequency pairs observed for iminodiacetonitrile can be attributed unambiguously to resonance frequencies of imino and cyan0 nitrogen atoms, respectively. Although two chemically different nitrogen atoms exist in a methylaminoacetonitrile molecule, only one set of resonance frequencies was observed in the frequency range of cyano nitrogen in aliphatic nitriles. Three resonance frequencies were observed for cyanoacetamide. The absorption curve of a line at 2367.7 kHz recorded by Zeeman modulation had a lineshape characteristic of v’ signals and had a sharp negative wing indicating a large asymmetry parameter. Since amide nitrogen atoms in formamide and urea are known to have a large asymmetry parameter (II, 12) this line can be attributed to the VI frequency of nitrogen in the amide group. The two remaining frequencies yielding a small asymmetry parameter can be assigned to cyano nitrogen atoms. Since all the compounds investigated are asymmetrically substituted derivatives of acetonitrile, they have no threefold axis of symmetry along the C = N bond even in an isolated molecule. In fact, observed asymmetry parameters of cyano nitrogen are fairly large. However, they are not very large compared with those of long-chain aliphatic nitriles, the asymmetry parameters of which are as large as 0.05 (2, 5). The nitrogen-14 quadrupole coupling constant of a cyano group having a cylindrical symmetry can be expressed by (I, 13) eQq = [s + i,(l

- s) - inI [leQq,l/
+

f&)1.

PI

Here the sp-hybridized a-bond orbital, the lone-pair orbital, and a n-bond orbital of nitrogen are assumed to be occupied by 1 + i,, 2, and 1 + in/2 electrons, respectively.

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285

The symbols s, eQqa, i, and E stand for the extent of s-character of the a-bond orbital, the quadrupole coupling constant of an electron in a 2p-orbital of nitrogen, the total ionicity (i,, + i,) of nitrogen, and the screening constant introduced by Townes and Schawlow (14), respectively. Since the s-character, s, can be assumed to be equal to 0.5 (sp-hybridization) for the a-bond orbital of nitrogen in cyano groups, this equation involves only two unknown parameters and indicates that the quadrupole coupling constant increases with increasing i, and with decreasing i,. The observed coupling constants of cyanoacetic acid, methyl cyanoacetate, ethyl cyanoacetate, and ethylene cyanohydrin are larger than the asymptotic value 3.80 MHz for the coupling constant of long-chain aliphatic nitriles (2). These compounds have a general formula, R’CH, CN. The electron-withdrawing power of R’ is stronger than that of alkyl groups because of the presence of electronegative oxygen atoms in R’. Therefore, the inductive effect of R’ gives rise to a decrease in n-electron polarization of the cyano group and decreases the n-electron density on the nitrogen atom, leading to a large quadrupole coupling constant. In these compounds, the foregoing effect seems to predominate over the inductive effect that may reduce i,. Acetone cyanohydrin shows three coupling constants. Although large coupling constants are anticipated for this compound because of the presence of a hydroxyl group on the x-carbon, one of the three coupling constants is extremely low, suggesting that intermolecular hydrogen bonding might be responsible for the low coupling constant. The existence of hydrogen bonds involving cyano nitrogen was expected also from the fact that the resonance signals yielding the low coupling constant are broad as compared with other signals yielding large coupling constants at all temperatures studied. The interpretation of observed coupling constants of cyano nitrogen in iminodiacetonitrile and methylaminoacetonitrile is difficult because of the possible existence of other effects leading to small quadrupole coupling constants. These compounds show relatively large asymmetry parameters of cyano nitrogen, suggesting the existence ofthe intermolecular interaction of a donor-acceptor type between lone-pair electrons in imino nitrogen and r-electrons in cyano groups as pointed out by Colligiani et al. (5). This interaction may increase the n-electron population and reduce the coupling constant of cyano nitrogen. In fact, iminodiacetonitrile yields a low coupling constant of cyano nitrogen in spite of the presence of an electronegative imino group on the a-carbon. The coupling constant of cyano nitrogen in cyanoacetamide is very small, suggesting the presence of a fairly strong intermolecular interaction. In order to calculate the ionic characters, in and i,-, of N-H and N-C bonds in iminodiacetonitrile, let it be assumed that an imino group has a tetrahedral configuration and that the maximum field gradient lies along the lone-pair orbital. Then the quadrupole coupling constant and the asymmetry parameter of imino nitrogen can be expressed by (15, 16) eQq = ;[l - 3(2i, + &)I leQq,j/(l + is), tJ= 4ji, - i&311 - (2i, + &J/31.

[31

Here the lone-pair orbital is assumed to be occupied by two electrons. The value of leQq,l has been estimated at 11.3 MHz (15), and E is assumed to be equal to 0.3, after Townes and Schawlow (14). From the observed coupling constant and asymmetry parameter along with the values estimated for leQq,l and E, iu and ic were calculated

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as 0.37 and 0.18, respectively. These values lead to the total ionicity, i = 2i, + iH, of the nitrogen atom equal to 0.73. This value is small compared with those of amino nitrogen in primary aliphatic amines (16) as well as those of imino nitrogen in dimethylamine (8) and saturated cyclic amines (9, ZO).l The inductive effect of two cyano groups in an iminodiacetonitrile molecule may be responsible for the low ionicity of imino nitrogen. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14.

R. S. S. S. A. R. A. P. A. A. L. S. R. C.

IKEDA, S. ONDA, D. NAKAMURA, AND M. KuBo,J. Whys. C/rem. 72,2501 (1968). ONDA, R. IKEDA, D. NAKAMURA, AND M. KUBO, Bull. Chem. Sot, Jap. 42,177l (1969). ONDA, R. IKEDA, D. NAKAMURA, AND M. KUBO, Bull. Chem. Sot. Jap. 42,274O (1969). ONDA, R. IKEDA, D. NAKAMURA, AND M. Kueo, Bull. Chem. Sot. Jap. 46,2878 (1973). C~LLIGIANI, L. GUIBB, P. J. HAIGH, AND E. A. C. LUCKEN, Mol. Phys. 14,89 (1968). IKEDA, D. NAKAMURA, AND M. KUBO, J. Phys. Chem. 70,3626 (1966). COLLIGIANI, L. GUIB~, P. J. HAIGH, AND E. A. C. LUCKEN, Mol. Phys. 19, 144 (1970). J. HAIGH, P. C. CANEPA, G. A. MATZKANIN, AND T. A. SCOTT, J. Chem. Phys. 48,4234 TZALMONA, Phys. Left. 20,478 (1966). COLLIGIANI, R. AMBROSETTI, AND R. ANGELONE, J. Chem. Phys. 52,5022 (1970). GUIBB AND E. A. C. LUCKEN, C. R. Acad. Sci. Paris 263B, 815 (1966). KOJIMA, M. MINEMATSU, AND M. TANAKA, J. Chem. Phys. 31,271 (1959). IKEDA, D. NAKAMURA, AND M. KUBO, J. Chem. Phys. 72,2982 (1968). H. TOWNES AND A. L. SCHAWLOW, “Microwave Spectroscopy,” p. 225, McGraw-Hill, York, 1955. 15. R. IKEDA, S. NODA, D. NAKAMURA, AND M. KUBO, J. Magn. Resonance 5, 54 (1971). 16. S. ONDA, H. HARADA, D. NAKAMURA, AND M. KUBO, J. Magn. Resonance 8,238 (1972). 1 and the and

By use of the same procedure described in the text, we have calculated ic, iH, and i as 0.42, 0.24, 0.91, respectively, for dimethylamine and as 0.42,0.28, and 0.97, respectively, for cyclic amines on basis of the values of eQq and q reported in the literatures (8-10). Here the average values of eQq q were taken over five saturated cyclic amines having a six-membered ring.

(1968).

New