J. Phys. Chem. 1987, 91, 6595-6600 the orbital on 0 1 containing unpaired spin density.I6 With Bo = 14 MHz, B2 = 263 MHz, and p = 0.17, the value of 4 is 70'. If the vector P is used as the symmetry axis of the orbital on 0 1 containing unpaired spin density, the calculated value of 4 is 24O. This large difference, 70' versus 24', indicates that a significant portion of p(07) lies in the plane normal to P. The hfc to H 5 is observed in aMeMan but not PMeGal. Although H5 is t to the main spin population on C7, it is 6 to the spin density on 0 1 . Hfc's of the 6 type have been observed for oxygen-centered radi~a1s.I~One can view the isotropic component of 'A as arising from hyperconjugation of the spin density from the 0 1 orbital to the C1-05 bond and from the C1-05 bond to the C5-H5 bond. These torsion angles, give in Table 11, are slightly less favorable in PMeGal than in aMeMan. More important to explaining the lack of H5 hfc in PMeGal is the expected (16) Bernhard, W. A,; Close, D. M.; Huttermann, J.; Zehner, H. J. Chem. Phys. 1977,67, 1211. (17) Snow, L. D.; Williams, F. Faraday Discuss. Chem. Sac. 1984,78, I.
6595
smaller dipolar hfc component. The ratio of C7- - -H5 distance in OMeGal to that in aMeMan taken to the sixth power is 0.59; thus, the dipolar component in PMeGal should be -0.6 times that of aMeMan. Conclusions The C H 2 0 R radical in aMeMan is bent by an angle 0 = 5.8 f 0.4'. Other angles are H-C-H' = 127.0 f 0.4', 0-C-H = 108.0 f 0.2', and 0-C-H' = 122.1 f 0.2'. The unpaired electron densities are pc E 0.83 and po E 0.17.
Acknowledgment. We thank Robert Spalletta for writing the software that was used in data collection. This research was supported in whole by DOE Grant DE-FG02-85ER60282 with the US.Department of Energy a t the University of Rochester, Department of Biophysics, and has been assigned Report No. DOE/EV/03490/2533. Such support does not constitute an endorsement by DOE of the views expressed in this article. Registry No. cuMeMan, 617-04-9; cuMeMan radical, 1 1 1025-82-2.
Vibrational and Electronic Spectra of the H 4- HCN Reaction Products Trapped in Solid Argon Marilyn E. Jacox Molecular Spectroscopy Division, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: April 21, 1987; In Final Form: June 29, 1987)
The reaction of HCN isolated in solid argon with H atoms formed by photolysis or by a discharge leads to the stabilization of a sufficient concentration of H2CN for observation of both its vibrational and electronic absorption spectra. Detailed isotopic substitution experiments have led to a vibrational assignment for this species. The analysis of the vibrational data is consistent with a planar molecular structure and with partial triple bond character for the CN bond. The previously reported electronic absorptions of HzCN have been reproduced, with only a small matrix shift. The appearance of more highly excited members of the electronic band system suggests a tentative assignment of the band structure for H,CN-d,. An infrared absorption that disappears on irradiation of the sample by visible light behaves appropriately for assignment to the CNH bending mode of cis- or trans-HCNH.
Introduction Because both H C N and H atoms are important species in the combustion of nitrogen-containing organic molecules and in hydrocarbon-air flames, their primary reaction product is expected to play a significant role in the mechanism by which chemically bound nitrogen is converted to nitrogen oxides. One potential product, H2CN, has also been postulated to be formed in the early stages of the decomposition of the nitramines,I an important class of high-energy molecules. However, relatively little is known about the properties of the H2CN free radical and almost nothing about the other isomeric C H 2 N species. The first detection of H2CN was that by Adrian and coworkers,2 who obtained its electron spin resonance spectrum in argon-matrix studies of the mercury-arc photolysis of Ar:HCN:HI mixtures and of the 122-nm photolysis of Ar:HCN samples. Evidence was presented for the presence of two equivalent hydrogen atoms. This conclusion was confirmed in a later electron spin resonance study by Banks and Gordy3 of the reaction of H atoms produced in a discharge with solid HCN. The electron spin (1) Melius, C. F.; Binkley, J. S. Proceedings of the 21st International Symposium on Combustion; Combustion Institute: Pittsburgh, PA, 1987; in press. (2) Cochran, E. L.; Adrian, F. J.; Bowers, V. A. J . Chem. Phys. 1962,36, 1938. (3) Banks, D.; Gordy, W. Mol. Phys. 1973,26, 1555.
resonance spectrum of HzCN has also been detected in various radiolysis systems4+ and in the low-temperature pyrolysis of the nitramine HMX.7 A pair of absorption bands that appeared at 281-285 nm on flash photolysis of formaldazine, (H2CN),, was attributed* to H2CN, an assignment that was supported by the appearance of weak bands in these same positions on flash photolysis of formaldoxime, H2C=NOH.9 Higher resolution flash photolysis studiesl0JI of normal and deuterium-substituted formaldazine demonstrated the presence of two hydrogen atoms in the molecule and yielded short progressions for the deuteriated species. However, the bands were too diffuse for the appearance of rotational structure. The structure of HzCN has been explored in a number of a b initio c a l c ~ l a t i o n s , l ~which - ~ ~ concur in predicting that the ground ~~
(4) Behar, D.; Fessenden, R. W. J . Phys. Chem. 1972,76,3945. (5) Fujiwara, M.; Tamura, N.; Hirai, H. Bull. Chem. SOC.Jpn. 1973,46, 701. (6) Symons, M. C. R. Tetrahedron 1973, 29, 615. (7) Morgan, C. U.; Beyer, R.A. Combust. Flame 1979,36,99. (8) Ogilvie, J. F.; Horne, D. G. J . Chem. Phys. 1968,48,2248. (9) Horne, D. G.; Norrish, R. G. W. Proc. R. SOC.London, A 1970,315, 287. (10) Horne, D. G.; Norrish, R. G. W. Proc. R. Soc. London, A 1970,315, 301. (11) Ogilvie, J. F. Can. J . Spectrosc. 1974,19,89. (12) Hinchliffe, A. J . Mol. Strucr. 1980,67,101.
This article not subject to US.Copyright. Published 1987 by the American Chemical Society
6596 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
state of the molecule should be planar. Most of these calculations suggest that the C H bond length should be typical and that the C N bond should possess partial triple bond character. LohrI5 has also calculated a stable potential minimum for the trans-HCNH isomer, and Bair and Dunning" have found a potential minimum for the somewhat less stable cis-HCNH species, as well. These latter workers have considered in detail the barriers and product energies for the H H C N reaction. Recently, Wagner and Bair'* have reported further calculations of the low-pressure H + H C N reaction to produce H2 CN. In an attempt to explain the band intensity pattern of the flash photolysis experiments, which suggested that H2CN possesses two electronic transitions separated by only 600 cm-', Adams and c o - w ~ r k e r shave ' ~ calculated the vertical electronic structure of H2CN. Their results suggest that four excited states should lie between 33 000 and 43 000 cm-' but that only one of them has symmetry (2A1) appropriate for an allowed transition from the ground state (2B2). It was suggested that the second H2CN band may be contribged by a vibronically allowed but electronically forbidden 2Bl-X *B2 transition. The stabilization of H2CN in the early argon-matrix electron spin resonance studies2 of the H H C N reaction suggests that it may be possible to obtain the vibrational spectrum of HzCN by using infrared detection. Furthermore, the calculation of stable potential minima for cis- and trans-HCNH suggests that such experiments may yield spectroscopic data for these heretofore unidentified species. Observation of the electronic spectrum of H2CN formed by the H H C N reaction in solid argon would preclude the contribution of relatively complex molecular fragments or of hot bands to the spectrum. The results of such experiments are presented in this paper.
+
+
+
+
Experimental Details20 The procedures used for the preparation of the HCN, H13CN, and HCISNsamples were the same as those previously reported.2' For a few experiments, an equimolar HCN:D20 sample was condensed into a small volume and allowed to stand overnight. After dilution of the product with sufficient argon to give the desired mole ratio, the water was removed by passing the sample through a column packed with PZOS. A moderate amount of deuterium exchange was observed. Three different H-atom sources were used in these experiments. The first of these was the photolysis of HI with the full light of a medium-pressure mercury arc. In those experiments, Ar:HCN samples of mole ratio ranging from 25 to 50 were codeposited with an approximately equal amount of Ar:HI sample of mole ratio from 50 to 100. In order to avoid filtering of radiation from the inner layers of the deposit, after the infrared spectrum of a small amount of the unphotolyzed sample had been established, photolysis was conducted concurrently with the deposition. The second H-atom source was H C N itself, produced by the vacuum-ultraviolet photolysis of the Ar:HCN sample, with a mole ratio ranging from 100 to 400, by 147-nm xenon resonance radiation or by 122-nm hydrogen resonance radiation. The use of these radiation sources has previously been described.22 In some of the studies, H C N was dissociated by codepositing the Ar:HCN sample with a beam of argon atoms excited in a low-power microwave discharge. The experimental configuration for these studies has also previously been described.23 The third source of H atoms was provided by conducting the (13) So, S. P. Chem. Phys. Lett. 1981,82, 370. (14) Barone, V.; Lelj, F.; Russo,N.; Ellinger, Y.; Subra, R.Chem. Phys. 1983, 76, 385. (15) Lohr, L. L. J . Phys. Chem. 1985, 89, 3465. (16) Koch, W.; Frenking, G. J . Phys. Chem. 1987, 91, 49. (17) Bair, R. A.; Dunning, T. H., Jr. J . Chem. Phys. 1985, 82, 2280. (18) Wagner, A . F.; Bair, R.A. Int. J . Chem. Kinet. 1986, 18, 473. (19) Adams, G. F.; Yarkony, D. R.;Bartlett, R. J.; Purvis, G. D. Inr. J . Quantum Chem. 1983, 23,437. (20) Certain commercial instruments and materials are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the instruments or materials identified are necessarily the best available for the purpose. (21) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1967, 47, 278. (22) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1967, 47, 5146. (23) Jacox, M. E. Chem. Phys. 1975, 7, 424
Jacox microwave discharge through an Ar:H2 (or D2) = 200 sample instead of through pure argon. In both of the discharge sampling configurations, a pinhole in the end of the discharge tube served as a baffle to inhibit the backstreaming of the H C N into the discharge region. All of the sample deposits were maintained at a temperature of 14 K. After the spectrum of the initial sample deposit had been recorded, the sample was exposed to the full or filtered light of a medium-pressure mercury arc, in order to determine the threshold and products of secondary decomposition. Corning filters of glass type 3384, 3389, 7380, and 7740, with short wavelength cutoffs of 490, 420, 345, and 280 nm, respectively, were used in these studies. Infrared observations on systems in which H atoms were produced by ultraviolet or vacuum-ultraviolet photolysis were conducted with a Beckman IR-9 spectrophotometer and a reflection sampling configuration that has previously been described.23 Under typical scanning conditions, the resolution and relative and absolute frequency accuracies are estimated to be 1 cm-' between 400 and 2000 cm-I and 2 cm-' between 2000 and 4000 cm-I. Most of the spectra in the discharge sampling experiments were obtained at a resolution of 0.2 cm-' by using a Bomem DA3.002 interferometer with an optical configuration that has previously been described.24 Survey scans of the near-infrared and visible spectral regions were also conducted with this instrument. Ultraviolet absorption spectra were obtained with a 0.75-m Ebert scanning monochromator. Details of this experimental configuration have previously been given.**
Observations Infrared Studies. The Ar:HCN samples used for the preliminary studies in which mercury-arc photolysis of H I served as the H-atom source contained between 1% and 2% of HCN, leading to the appearance of prominent absorptions not only of dimeric HCNZSbut also of higher aggregates. On mercury-arc photolysis, a weak to moderately intense absorption, assigned to the C N free radical:' appeared at 2046 cm-I, and a weak to moderately intense, unidentified product absorption appeared at 1338 cm-I. The Ar:HCN samples used for the vacuum-ultraviolet photolysis studies were considerably less concentrated, and the absorptions of H C N aggregates were greatly reduced in intensity. As has previously been reported,21prominent absorptions of H N C and a moderately intense absorption of C N at 2046 cm-' were typical of these experiments. In addition, moderately intense product absorptions appeared at 1336 and 3104 cm-' and a weak product absorption at 886 cm-I. Several experiments were conducted in which an Ar:HCN = 200 sample was codeposited with a beam of argon atoms that had been excited in a microwave discharge. Again, the absorptions of H N C were prominent, and peaks at 886, 1336, and 1304 cm-' appeared with moderate intensity. A new absorption appeared at 954 cm-', on the shoulder of the relatively broad 950-cm-' H N C 2v2 absorption. At the higher resolution of the Bomem studies, the very sharp peaks of H N C at 2032 cm-' and of C N at 2046 cm-' were greatly intensified. Weak absorptions were also detected at 912.8 and 1725.4 cm-I in the Bomem observations of this system, which afforded a much decreased noise background. When a low-power microwave discharge through an Ar:H2 sample was used as the H-atom source, these same product absorptions appeared with intensities similar to those in the excited argon atom experiments, and there were no new peaks. The spectra of the three most prominent new peaks, obtained at 0.2cm-' resolution, are shown in traces a of Figure 1, and the positions and approximate relative intensities of all of the product absorptions except C N and H N C are summarized in the first column of Table I. There was no evidence for absorptions that could be assigned to CH2NH or CH3N.26-28 (24) (25) (26) (27)
Jacox, M. E.; Olson, W. B. J . Chem. Phys. 1987, 86, 3134. Pacansky, J. J . Phys. Chem. 1977, 81, 2240. Milligan, D. E. J . Chem. Phys. 1961, 35, 1491. Jacox, M. E.; Milligan, D . E. J . Mol. Specfrosc. 1975, 56, 333.
-
Spectra of H
3125
+ H C N Reaction Products
3100
3075 1350
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6597 I
TABLE I: Infrared Absorptions' (cm-') Contributed by Photosensitive Products of the H HCN Reaction in Solid Argon
+
H+ HX2Cl4N 886.3 m 912.8 w 954.1 w, sh
1336.6 m
(50%)
885.8 m 906.1 vw 912.7 vw 945.2 w, sh 954.1 w, sh
1725.4 w
1336.5 m 1691.2 vw 1725.4 vw
3103.5 m
3090.4 wm 3103.2 wm
H+ D+ HL2C15N HI2Cl4N 776 w 884.7 wm
D+ D12C14N 776 w
910.5 vw 953.6 w
1073.4 w 1208.0 w
1073.4 w 1208.0 w
1333.6 m
2427.5 wm 2427.5 wm 3102.9 m
"vw,very weak; w, weak; m, medium; sh, shoulder. Absorptions observed for the H + HCN reaction were also present in D + HCN and D + DCN studies.
A series of filtered photolysis experiments has demonstrated that a t least two different species contribute the unassigned absorptions in these experiments. The 886-cm-' peak behaved uniquely; it diminished in intensity when the deposit was exposed to filtered mercury-arc radiation of wavelength longer than 420 nm and was readily destroyed by radiation with a short wavelength cutoff of 345 nm. This behavior would inhibit stabilization of the carrier of this absorption in the studies of the mercury-arc photolysis of Ar:HCN Ar:HI deposits. Exposure of the deposit to mercury-arc radiation of wavelength shorter than 280 nm led to a decrease in the intensities of the remaining absorptions summarized in Table 1. Presumably the HI and I2 in the Ar:HCN Ar:HI experiments absorbed a sufficient fraction of the mercury-arc radiation to prevent complete destruction of the carrier of the 1336-cm-l absorption. The HNC absorptions were unchanged on irradiation of the sample by the full light of the medium-pressure mercury arc. Isotopic substitution experiments have provided additional data useful in the assignment of the photosensitive product absorptions. These studies were conducted with discharged Ar:H2 as the H-atom source, in order to provide a capability for introducing D atoms into the system. The Bomem interferometer was also
+
+
(28) Carrick, P. G.; Engelking,
P. C. J . Chem. Phys.
I
I
075 cm-1
1325 900
Figure 1. (a) 3.96 mmol of Ar:HCN = 200 codeposited at 14 K over a period of 323 min with 5.35 mmol of discharged Ar:H2 = 200. (b) 4.71 mmol of Ar:HCN (-50% I3C) = 200 codeposited at 14 K over a period of 255 min with 4.17 mmol of discharged Ar:H2 = 200. (c) 5.24 mmol of Ar:HCISN = 200 codeposited at 14 K over a period of 312 min with 4.28 mmol of discharged Ar:H2 = 200.
H+ H"C14N
I
1984, 81, 1661.
300 280 260 nm Figure 2. (a) 4.07 mmol of Ar:HCN = 100 deposited at 14 K over a
period of 93 min with concurrent photolysis by hydrogen-discharge radiation. (b) 4.28 mmol of Ar:HCN = 200 codeposited at 14 K over a period of 353 min with 2.14 mmol of discharged Ar. used for the isotopic substitution experiments, in order to optimize the precision and sensitivity of the observations. As is evident from traces b and c of Figure 1 and from the vibrational frequency summary given in the second and third columns of Table I, many of the heavy-atom isotopic shifts were small. However, in the study that used a H C N sample enriched to approximately 50% in carbon-13 there was a 13-cm-' splitting in the 3103-cm-' absorption, demonstrating that it is contributed by a CH-stretching vibration. The very weak 1725.4-cm-' peak was accompanied by a similar peak at 1691.2 cm-', suggesting the assignment of this absorption to a CN-stretching vibration. Unfortunately, the product yield was sufficiently lower in the studies on the more dilute Ar:HCI5N sample to prevent the detection of the nitrogen-15 counterpart of this absorption. There was also almost a 10-cm-' splitting in the 954-cm-' peak. Since the carbon-13 shift in the 477-cm-' bending fundamental of H N C has been observed2' to be less than 1 cm-I, the 954-cm-' peak cannot be assigned to the bending overtone of H N C trapped in a site in which it can interact with a photosensitive molecule. In studies of the reaction of H C N and DCN with D atoms produced by a discharged Ar:D2 sample, a few relatively weak absorptions with photodecomposition thresholds near 280 nm were detected. These are also summarized in Table I. Ultraviolet Studies. A search for low-lying electronic absorptions in the 4000-8000- and 10 000-20 000-cm-' spectral regions was conducted by using the Bomem interferometer and Ar:HCN samples codeposited with discharged Ar:H2. However, no absorptions were found. Because of the potential interference from visible-ultraviolet absorptions of HI and its photodecomposition products, the first ultraviolet absorption studies were conducted with the 122-nm photolysis of an Ar:HCN sample, a system that had yielded a prominent electron spin resonance signal of C N and a weak signal of H2CN.* As is shown in trace a of Figure 2, when an Ar:HCN = 100 sample was deposited with concurrent 122-nm photolysis, several new absorptions appeared between 290 and 260 nm,in addition to the very prominent C N absorptions. The positions of these new absorptions are summarized in Table 11. In this particular experiment, a prominent OH absorption, presumably a product of the vacuum-ultraviolet photolysis of traces of water desorbed from the walls of the vacuum system, also appeared at 31 1 nm. Therefore, a portion of the abso_rption_near285 nm is contributed by the 1-0 band of the OH A2Z+-X211 transition. The two most prominent peaks, at 34990 and 35 436 cm-', lie close to the two peaks that have previously been assigned8-l' to gas-phase
6598
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
TABLE 11: Electronic Absorption Bands (cm-') of H,CN-d, H2CN Ar
gasa
35075
oT
34990
~~~~~~
35620
35817w,sh
35817w,sh
1883
1437 36403 1361
36873 1361
37764
37764
D2CN
HDCN gasa
gasa
3 5 4 8 1 7 35696 35550-_j1265 36310 1894 36115 3
6
9
3
0
1
"Ogilvie, J. F. Can. J. Spectrosc. 1974, 19, 89.
H,CN, at 35 075 and 35 620 cm-'. Ultraviolet absorption observations were also conducted on Ar:HCN = 200-800 samples d e p o s i t e d with a beam of excited argon atoms. In a study using undischarged argon, no peaks were observed in the 190-580-nm spectral region. When the argon beam was excited in the microwave discharge, very prominent C N absorptions resulted. In addition, weak to moderately intense absorptions appeared between 260 and 290 nm. The two most prominent of these peaks could be detected even in the Ar:HCN = 800 study. In the experiment for which this spectral region is shown in trace b of Figure 2 the 3 11-nm OH absorption was below the limits of detection. Except for increased optical scattering at the shorter wavelengths, the spectrum observed in this system was in good correspondence with that of the 122-nm photolysis experiments and demonstrated that in the argon matrix, as in the gas phase, the longest wavelength H2CN absorption is somewhat less intense than the shorter wavelength band. The absorptions between 260 and 290 nm were unchanged on prolonged exposure of the deposit to mercury-arc radiation of wavelength longer than 280 nm. However, all of these bands were destroyed upon irradiation of the sample with the full light of the medium-pressure mercury arc.
Discussion Spectrum of H2CN. The correspondence of the two strongest bands in the electronic spectrum with the gas-phase bands previously assigned to H2CN is sufficiently good to support the assignment of these bands to the same species in both experimental systems. The infrared absorptions of H N C are unchanged on exposure of the argon-matrix samples to the full light of the medium-pressure arc, whereas the carrier of these peaks photolyzes. Therefore, assignment of one or both of these peaks to HNC is excluded. Since the matrix experiments are chemically simpler than the previous gas-phase systems in which these bands were identified, their assignment to a species of formula C H z N is supported. The calculations of Bair and Dunning1' indicate that H2CN should be the most stable product of the H + H C N reaction and that the barrier to its formation should be lower than that for the formation of any other product; they estimate the barrier height to be 4 kcal/mol and the formation of H2CN to be exothermic by 25 kcal/mol. The appearance of both of the bands at 14 K excludes the possible contribution of the lower frequency absorption by a hot band, and the observation of photodissociation is consistent with the report that the gas-phase bands are diffuse." The observation of the electronic spectrum of H2CN in the matrix studies suggests that infrared absorptions of this species may also be detectable. The photolysis behavior of the peaks at 913, 954, 1336, 1725, and 3103 cm-' parallels that of the ultraviolet absorptions of H2CN, consistent with the contribution of
Jacox the infrared absorptions by that species. The detection of only the 1336-cm-' peak in the Ar:HCN + Ar:HI experiments does not preclude the assignment of the remaining peaks to the same product, since the 3100-cm-' spectral region was partially obscured by the broad absorption of H C N aggregates and the other peaks would have been too weak to be detected. Further support for the assignment of these five absorptions to H2CN has been obtained from consideration of the isotopic shifts. In the following discussion the Wilson GF-matrix formulation of the normal-coordinate problem29is used, together with the structure obtained for H2CN in the ab initio calculations by Bair and Dunning." In the high-frequency approximation, discussed by H e r ~ b e r g , ~ ~ each of the two CH-stretching vibrations of H2CN is separable from the lower frequency modes having the same symmetry. Thus, each of the CH-stretching frequencies of H2CN should be approximately proportional to the square root of the CH-stretching G-matrix element of the appropriate symmetry. If the 3103.2-cm-' peak is assigned to v1 (al), the symmetric CH-stretching mode of H2CN, the corresponding mode of the carbon-13 species is calculated to lie at 3098.4 cm-I, in poor agreement with the observed absorption at 3090.4 cm-l. On the other hand, if the 3103.2-cm-' peak is assigned to v5 (bJ, the antisymmetric CHstretching fundamental, the calculated position of the corresponding carbon-13 species absorption is 3089.8 cm-l, in excellent agreement with the observed peak. Similar calculations help to exclude an alternate assignment of the 3 103-cm-' peak to cis- or tram-HCNH. The G-matrices of these two species have also been calculated with the Bair and Dunning s t r ~ c t u r e s . 'Although ~ the high-frequency approximation would not separate the NH- and CH-stretching modes, these vibrations do not share a common atom, so that GI2 = 0. If F12,the stretching-interaction force constant, is small, the two vibrations should be approximately separable. For either rotamer, the calculated position of the ')CH-stretching fundamental is 3093.9 cm-], in poor agreement with the observed value. The weak absorption at 1725 cm-l is in the expected region for the CN-stretching fundamental of H2CN. Moreover, the 34-cm-' shift on carbon- 13 substitution is of appropriate magnitude for assignment of the 1725-cm-I peak to v2 (al). The 1336-cm-' absorption falls in the spectral region appropriate for the CH2 ''scissors" fundamental, vj (a,). Although at first glance a significant carbon- 13 shift might be expected for this vibration, studies in this laboratory have shown that the carbon-1 3 shift in the corresponding mode of HzCO is less than 1 cm-'. This vibration would be expected to have a large shift on deuterium substitution. The ratio 1074/1336 = 0.80, a suitable value. Furthermore, the 1208-cm-' peak of the singly deuterium-substituted species is situated very close to the mean of the H2CN and DzCN absorptions, as would be expected for this assignment. The out-of-plane deformation fundamental of H,CN, u4 (bl), is the only vibration of this symmetry. Therefore, its isotopic frequency ratios are readily calculated from the G-matrix values. This fundamental of H 2 C 0 lies31 at 1167 cm-', suggesting that the 954-cm-l H2CN peak may be contributed by the out-of-plane deformation. The observed and calculated positions of this vibrational fundamental for the various isotopically substituted species of H2CN are summarized in Table 111. The agreement for the carbon- 13 and nitrogen- 15 substituted species is excellent. Since a definitive structure was not available for the calculations and since there is expected to be a significant anharmonic correction on deuterium substitution, the correspondence between the calculated v4 (b,) DzCN fundamental and the weak absorption observed at 776 cm-' in the deuterium atom reaction studies is sufficiently good to suggest assignment of that absorption to u., (b,) of D,CN. (29) Wilson, E. B., Jr.; Decius, J. C.; Cross, P. C. Molecular Vibrarions; McGraw-Hill: New York, 1955; pp 54-145. (30) Herzberg, G. Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand: Princeton, NJ, 1945; pp 195-201. (31) Reference 30, p 300.
Spectra of H
+ H C N Reaction Products
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6599
TABLE 111: Vibrational Assignment of Infrared Absorptions (cm-I) of HZCN
H,I2Cl4N H,”CI4N H,12C’5N D,I2Cl4N CHI sym stretch calcd CN stretch
VI
(a11
u2
(al)
obsd
calcd CHI Yscissorsn v j (al) obsd
calcd out-of-plane
u4
v5
2992.1
2999.3
2225.8
1725.4 1725.4
691.2 691.3
1700.8
655.7
1336.6 1336.7
336.5 336.5
1333.6 1333.4
073.4 972.4
954.1 954.1
945.2 944.8
953.6 953.5
776 760.8
3103.2 3103.5
3090.4 3089.6
3102.9 3103.5
2427.5? 2321.0
912.8 912.7
906.1 906.4
910.5 910.3
716.4
(b,)
obsd
calcd CH2 asym stretch obsd calcd CH2 rock obsd calcd
2999.5
(b2)
v6 (b2)
TABLE IV: Force Constants’ of HICN
FCH FCN FCH2
FHCN
5.07 11.73 0.44 0.43
FCN.CH2 FCH2,HCN
FOPLA
-0.32 0.03 0.24
Stretching force constants X lo2 Nsm-l; bending and bending interaction force constants X 10l8 N-m; stretch-bending interaction force constant X10-* N. The only other fundamental of H2CN expected to fall in the spectral region near 1000 cm-’ is the in-plane CH2rocking mode, us (bz). The 912.8-cm-’ peak is a suitable candidate for assignment to this vibrational mode. The preliminary analysis of the infrared data given in the preceding discussion is consistent with the assignment of these five absorptions to H2CN. Further analysis of the vibrational spectrum has been conducted for the four isotopic species of HzCN which possess C2, symmetry by using the least-squares force constant adjustment program FADJ, developed by Scha~htschneider.~~ A diagonal valence force field was used for the b2 symmetry block, together with the assignment of the absorptions at 3103.2 and 912.8 cm-’ to the two fundamentals of this symmetry. As is shown in Table 111, this assignment leads to excellent agreement between the observed and calculated frequencies for both H2I3CN and H2C15N. The resulting values of FCH and F H C N are given in Table IV. Because of the large difference between the anharmonic correction for the C D stretching and that for the CHstretching vibrations, the calculated value for u5, the CD2 antisymmetric stretching fundamental, is expected to be a lower bound for the position of this absorption. The ratio 2428/3103 = 0.78, a value consistent with the assignment of the 2427.5-cm-’ absorption to u5 (b,) of D2CN. However, this assignment is not definitive, since in the one experiment conducted on the D + HI3CN experimental system no carbon- 13 splitting was observed for the 2427.5-cm-’ peak. It may be noted that the first overtone of the 1208-cm-’ absorption of HDCN should lie near 2416 cm-I and may be shifted and intensified by interaction with a nearby CD-stretching fundamental of this species, for which both absorptions would be of a’ symmetry. The calculations for the a, symmetry block were conducted with the values for the CH-stretching force constant and the H C N bending force constant fixed equal to those obtained in the b2 symmetry block calculations. Frequencies for the deuteriumsubstituted species were not included in the fit. The coefficients of the off-diagonal terms in the potential energy distribution were small. Furthermore, the average deviation between the observed (32) Schachtschneider, J. H. “Vibrational Analysis of Polyatomic Molecules”; Technical Report Nos.231-64 and 57-65; Shell Development Co.: Emeryville, CA, 1974, and private communication.
and calculated frequencies for H13CN and HCI5N was 0.1 cm-’ for the potential constants summarized in Table IV, strongly supporting the proposed assignment. However, the calculated frequency of u3 (al) of D2CN is 100 cm-’ lower than the observed value. As for the other modes involving large deuterium atom amplitudes, this deviation may result in part from the lack of an experimental molecular structure and from the omission of some off-diagonal terms from the vibrational potential. However, large anharmonic contributions to the vibration are likely to play the predominant role; the calculations of Bair and Dunning17indicate that the depth of the potential minimum for HzCN with respect to H + H C N is only about 25 kcal/mol. In general, the values of the force constants of H2CN are similar to those reported for related molecules. The value obtained for FCN, 11.73 X lo2 Nem-’, is intermediate between values typical as had been predicted by the ab initio of C=N and C=N bond~,3~
calculation^.^^-'^ The observation of additional structure in the electronic spectrum of HzCN isolated in solid argon permits further consideration of the assignment i n this_spectral region, as well. The absorption spectrum of the CZAl-X2B2transition of F2CN, expected to have a closely related electronic structure, has been observed near 360 nm by Dixon and c o - w o r k e r ~ . All ~ ~ of the data were consistent with a planar structure, suggesting that the lowest 2Al state of H2CN should also be planar. Short progressions in all three F2CN vibrations of a, symmetry were observed. Since band spacings of 1079 and 1265 cm-’ are evident in the gas-phase spectra” of D2CN and HDCN, respectively, it is suggested that these two band spacings are contributed by excitation of the CD2 and CHD “scissors” fundamentals and that the 1413-cm-’ spacing in the argon-matrix spectrum of H2CN results from the excitation of u3, the CH2 “scissors” deformation of H2CN. Two possible assignments are suggested in Table 11. For both of them the band origins of the ZAlstate and of the second, vibronically allowed transition are separated by approximately 446 cm-I. In the first assignment, the CHI “scissors” vibrations of both electronic transitions, with band spacings of approximately 1413 and 1437 cm-I, are excited. Alternatively, the 36873-cm-’ absorption may be contributed by the excitation of one quantum of the CNstretching vibration of the 2Al state. This second assignment is favored both because the need to invoke vibronic activation is minimized and because the CN-stretching vibration frequency would, within the experimental error, agree with the band spacing of 1894 cm-I here proposed for the CN-stretching vibration frequeccy of D2CN. The CN-stretching fundamental of F2CN in the C2Al state is increased by more than 100 cm-’ from the ground-state v a l ~ e . ~The ~ .assignment ~~ of the 1883-cm-’ H2CN band spacing to excitation of the CN-stretching mode would also exemplify parallel behavior for the corresponding transitions of H2CN and FzCN. For either assignment, the 37 764-cm-’ absorption, 1361 cm-’ above the peak attributed to excitation of the CH2 “scissors” fundamental, may be attributed either to excitation of a second quantum of this vibration or to excitation of u1 (al), the CHI symmetric stretching vibration, 2774 cm-’ above the band origin. Although the proposed assignments provide a useful framework for understanding the electronic spectra of the H2CN-d, species, they cannot be regarded as definitive. Unassigned bands are present for both H2CN and D2CN. Moreover, for both of the suggested assignments the first band of the allowed 2A1-2Bz transition, at 34990 cm-l, is somewhat less intense than the 35 436-cm-I absorption, tentatively assigned to a vibronically allowed transition. However, addition of the intensities of the higher vibrational bands of the 2Al state may well reverse the relative intensities of the two transitions. Spectra of Other Products. The calculations of Bair and Dunning17 indicate that there is a barrier of approximately 10 kcal/mol to the formation of cis-HCNH in the H H C N re-
+
(33) Reference 29, p 175. (34) Dixon, R. N.; Duxbury, G.; Mitchell, R. C.; Simons, J. P. Proc. R. SOC.London, A 1967, 300, 405. (35) Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1968, 48, 4040.
J. Phys. Chem. 1987, 91, 6600-6606
6600
action, which is exothermic by approximately 6 kcal/mol. The subsequent rearrangement of cis-HCNH to trans-HCNH would have a barrier of 8 kcal/mol and would be exothermic by approximately 5 kcal/mol. Rearrangement of trans-HCNH to H2CN, while exothermic by some 14 kcal/mol, would have a relatively high barrier, near 38 kcal/mol, exceeding the threshold for the formation of H 2 C N . Thus, the stabilization of trans-HCNH, and possibly also of the cis rotamer, would be expected in the matrix experiments. The absorption a t 886 cm-I, contributed by a product that is photolyzed by visible radiation, is a candidate for assignment to one of the two HCNH rotamers. For both of these species, Bair and Dunning" have calculated the torsion and the CNH bending vibrations to lie between about 900 and 1050 cm-I. The assignment to the torsion vibration is readily tested, since for either the cis or the trans structure this vibration is the only out-of-plane mode. With the structures calculated by Bair and Dunning and the observed value of 886.3 cm-I, the calculated positions of the torsion vibration for cis- and trans-HI3CNH are 881.7 and 884.2 cm-I, respectively. The corresponding values for cis- and transHCI5NH are 883.5 and 885.8 cm-'. The agreement with the observed frequencies, given in Table I , is not sufficiently good to support assignment to the torsion mode of either rotamer of HCNH. While data do not suffice for similar calculations of isotopic shifts in the CNH deformation (a') of the HCNH species, the observation that the shift on nitrogen-15 substitution is greater than that on carbon-13 substitution would be consistent with the assignment of the 886.3-cm-l absorption to the in-plane CNH deformation fundamental of either cis- or trans-HCNH. It cannot be determined from the present experiments whether HCNH results from a second channel for the H HCN reaction or from the addition of H atoms to HNC, a product of the cage recombination of H + CN.*' The appearance of the 2046-cm-' absorption of CN in the mercury-arc photolysis experiments, in which photodecomposition of HCN should not occur, may result from the photolysis of HzCN
+
+
+
to form H , C N . A substantial barrier is ~ a l c u l a t e d ' ~for~ 'the ~ direct H HCN reaction to form these products, which is endothermic. The ESR signal of CN was suppressed in the earlier studies2 of this system, which gave a prominent H2CN signal. A low-pressure mercury-arc photolysis source was used for the ESR experiments on Ar:HCN:HI samples, whereas a medium-pressure mercury-arc lamp was used for the present experiments. The low-pressure lamp may have had a considerably lower radiation output near 280 nm, where the strongest H2CN absorption occurs.
+
Conclusions H atoms react with HCN isolated in solid argon at 14 K to form HzCN in sufficient concentration for detection of five vibrational fundamentals and of extended structure in one of the previously reported electronic transitions. The analysis of the infrared spectra for isotopically substituted HzCN is consistent with a planar molecular structure with partial triple bond character for the CN bond. The ultraviolet observations support the previous identification of two electronic transitions of HzCN. A Librational assignment has been proposed for structure in the 2Al-XZB2band systems of the H2CN-d,,species. An infrared absorption at 886 cm-' can be assigned to the in-plane CNH deformation fundamental of cis- or trans-HCNH, which has a photodecomposition threshold in the visible region of the spectrum. The mechanism by which HCNH is formed cannot be established from this series of experiments. Acknowledgment. The design and construction of the transfer optics for the Bomem Fourier transfer spectrophotometer by W. Bruce Olson were crucial to the detection of the weaker H2CN absorptions in the studies using that experimental system. This work was supported in part by the US.Army Research Office under Research Proposal 2 1495-CH. Registry No. H atomic, 12385-13-6;HCN, 74-90-8;Ar, 7440-37-1; HZCN, 15845-29-1;H2I3Cl4N,110905-04-9;H2I2Ci5N,110905-05-0; D2I2Ci4N,51624-18-1.
Stability, Molecular Dynamics in Solution, and X-ray Structure of the Ammonium Cryptate [NH4+C2.2.2]PF,Bernard Dietrich, Jean-Pierre Kintzinger, Jean-Marie Lehn,* Bernard Metz,+ and Assou Zahidi Laboratoire de Chimie Organique Physique (UA 422). Institut Le Bel, Universitt Louis Pasteur, 67000 Strasbourg, France, and Laboratoire de Chimie MinZrale (UA 4051, EHICS Strasbourg. and Laboratoire de Cristallographie Biologique, IBMC, CNRS, 67000 Strasbourg. France (Received: March 27, 1987; In Final Form: June 16, 1987)
The macrobicyclic ligand 1 [2.2.2] forms a highly stable complex with the NH4+cation in aqueous solution. Although the spherical macrotricycle 2 forms an even stronger complex, 1 is a more efficient binder of NH4+at pH 7-8 because of easier protonation of 2. Crystal structure determination indicates that the complex is of cryptate type [NH4+C1],the NH4+substrate being held in the macrobicyclic cavity by NH+-.X hydrogen bonds with one bridgehead nitrogen and three oxygens. Determination of the 'H, I3C, 15N,and I4N NMR relaxation times and calculation of the corresponding correlation times allow a detailed description of the molecular dynamics of the NH4+cryptates of 1 and 2. The [NH,+Cl] cryptate shows weak dynamic coupling between the receptor and the bound substrate, which reorients rapidly inside the cavity. On the other hand, the [NH4+C2]cryptate shows strong dynamic coupling, the receptor and substrate having similar molecular reorientation times. Nuclear Overhauser effects indicate that the orientation of the NH4+cation inside the cavity is similar in solution to that in the crystal.
Complexes of polyatomic cations such as ammonium, guanidinium, hydrazinium, diazonium, hydronium, etc. have been intensively studied in particular because of the biological role of substituted ammonium ions. The parent ion, the ammoTUA 405 and IBMC.
0022-3654/87/2091-6600$01.50/0
nium cation NH4+,is of special interest for two major reasons: (i) NH4+is on the border between inorganic and organic chemCram, M. J. Acc. Chem. Res. 1978, 11, 8. (2) Lehn, J. M. Pure Appl. Chem. 1978, 50, 871. (3) Lehn, J. M. Pure Appl. Chem. 1979, 51, 919. (1) Cram, D. J.;
0 1987 American Chemical Society
