7613
J. Phys. Chem. 1991,95,7613-7617
Measurement of the Photoionization Spectra and Ionization Thresholds of the H2CN and D2CN Radicals F. L. Nesbitt,**+G. Marston,$L. J. Stief, Astrochemistry Branch, Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771
M. A. Wickramaaratchi, W. Tao, and R. B. Klemm Department of Applied Science, Brookhaven National Laboratory, Upton, New York 1 1 973 (Received: December 11, 1990; In Final Form: April 23, 1991)
The photoionization spectra of the H2CN and D2CN radicals were obtained by photoionization mass spectrometry (PIMS) using synchrotron radiation. The radicals were generated by the reaction of N with CH, and CD3, respectively. For both H2CN and D2CN a prominant feature was observed near 118.6 nm (10.5 eV) and the ionization threshold was determined to be 9.4 0.1 eV; both features provide additional signatures for identifying H2CN in complex systems. By use of a corrected value for AHf (H2CN) derived from a recent electron affinity measurement and other available measured or calculated thermochemical quantities for H2CN and HCNH radicals and radical ions, a value of 10.8 0.6 eV for the ionization energy of H2CN was derived. The much lower value derived for the ionization energy of HCNH (6.8 and 7.0 eV for the cis and trans isomers, respectively) is consistent with the product of the N + CH3 reaction being the H2CN isomer and not HCNH. The ionization threshold observed at 9.4 eV is attributed to autoionization arising from high Rydberg states of H2CN which couple into vibrationally excited states of the linear HCNH+ ground state of the ion. Also discussed are the roles of the H2CN radical and HCNH' radical ion in the chemistry of the atmosphere of Titan and in interstellarr clouds.
*
*
Introduction The H2CN radical is thought to be an important intermediate' in the decomposition of nitramines? the formation of H C N in the atmospheres of Titan3 and Jupiter,' laboratory studies of active nitrogen/hydrocarbon reactions: nitrogen/hydrmrbon chemistry in combustion! and the chemistry of circumstellar' and interstellar clouds.8 The radical ion is believed to play a significant role in the atmosphere of Titan9 and the ion-molecule chemistry of interstellar clouds.8 In recent experiments Marston, Nesbitt, and StiefIb developed a source of H2CN and employed low-energy electron impact mass spectrometry to detect both H2CN and D2CN and to monitor their reactions with atomic nitrogen and atomic hydrogen.lc HZCN(D2CN) N -.+ NH(ND) + HCN(DCN) (1) D2CN H H D DCN (2) Because of widespread interest in the H2CN radical and ion, and since very few fundamental properties of H2CN are known, it is of intererst to measure the ionization threshold and photoionization spectra of H2CN and D2CN with good precision. The techniques employed were electron ionization mass spectrometry (EIMS) and photoionization mass spectrometry (PIMS, argon resonance lamp and synchrotron radiation).
+
+
+
+
Experimental Section Experiments were performed in two different experimental systems. Measurements of the ionization threshold by electron ionization and photoionization (Ar lamp) were performed in a discharge flow/mass spectrometer at Goddard Space Flight Center (GSFC).'bs'o Measurements of photoionization spectra were performred in a discharge flow/photoionization mass spectrometer at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL)." All experiments were conducted at room temperature and at a total pressure of 1-2 Torr of helium. Flow velocities were 1000 cm/s at BNL and -2000 cm/s at GSFC. With the moveable injector typically at 5-10 cm from the sampling pinhole, reaction times were of the order of 5 ms.
-
'Research Associate, Chemistry Department, The Catholic University of America, Washington, DC 20064. NAS/NRC Postdoctoral Research Associate. Present address: Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX I 3QZ. U.K.
*
0022-365419 112095-7613SO2.5010 , I
,
H2CN and D2CN radicals were generated as described below, and spectra were obtained by using 750-pm slits (-0.3-nm fwhm resolution). In all the experiments reported here, a lithium fluoride window was employed to block second-order light from the monochromator. No corrections were made to the measured threshold values for the slit function of the monochromator, the internal energy of the HICN and DzCN radicals, or possible collision processes in the ionization region. Errors due to these effects should be less than the estimated uncertainties in the ionization thresholds ( f l . O eV for EIMS and f0.1 eV for PIMS). The H2CN radical was generated at the tip of a moveable injector by reacting N atoms with methyl radicals.'J2 N atoms were produced in a microwave discharge of N2/He, whereas methyl radicals were produced by the reaction of F atoms with CH4/He ( F atoms were produced in a microwave discharge of CF4/He). A similar procedure was used with CD4 to generate D2CN. In the experiments performed at GSFC, N and F were generated in the rear of the flow tube in separate microwave discharges. In the photoionization spectrum experiments performed ( I ) (a) Marston, G.; Stief. L. J. Res. Chem. Int. 1989, 12, 161. (b) Marston, G.;Nesbitt, F. L.; Stief, L. J. J . Chem. Phys. 1989, 91, 3483. (c) Nesbitt, F. L.; Marston, G.; Stief, L. J. J . Phys. Chem. 1990, 94, 4946. (2) Melius, C. F.; Binkely, J. S. Symp. ( I n t . ) Combust. [Proc.] 1986,21sf, 1953. (3) Yung, Y. L.;Allen, M. A.; Pinto, J. P. Asfrophys. J.,Supp/.Ser. 1984, 55, 465. (4) Kaye, J. A.; Strobel, D. C. Icarus 1983, 54,4176. ( 5 ) Safrany, D. R. Prog. Reacf. Kinef. 1971, 6 , I . Froben, F. W. Ber. Bunsenges. Phys. Chem. 1974,78, 1984. Brook, P. J.; Mile, 8. Chem. Commun. 1980, 395. (6) Glarborg, P.; Miller, J. A.; Kee, R. J. Combust. Name. 1986,65, 177. Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989. I S . 287. (7) Nejad, L. A. M.; Millar, T. J. Mon. Nor. R. Astron. SOC.1988. 230, 79. (8) Langer, W. D.; Graedel, T. E. Astrophys. J . , Suppl. Ser. 1989,69, 241. Prasad, S . S.;Huntress, W. T. Asfrophys. J . , Suppl. Ser. 1980, 43, 1. (9) Atreya, S. K. In Atmospheres and lotupheres offhe Outer Planets and their Safe//ites;Springer-Verlag: New York, 1986. (IO) Brunning, J.; Stief, L. J. J . Chem. Phys. 1986, 84, 4371.
( I I ) Klemm, R. 9.; Wickramaaratchi, M.A,; Gleason, J. F. Unpublished results. (12) Stief. L. J.; Marston, G.; Nava, D. F.; Payne, W. A.; Nesbitt, F. L. Chem. Phys. Left. 1988, 147, 570. Marston, G . ;Nesbitt, F. L.; Nava. D. F.; Payne, W. A.; Stief, L. J. J. Phys. Chem. 1989, 93, 5769.
Q 199 1 American Chemical Society
7614 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Nesbitt et al.
1
fE t
3J
2
8.0 -
H2CNSPECTRW
.
CH3THRESHOLD 0
5.0
\'
0'
4.0 -
\
4
3.0 -
0
2.02.01.0 -
110
114
iia
122
126
130
134
WAVELENGTH, nm
Figure 2. Photoionization spectrum of H$N. The photoion yield is the ion counts at m / e 28 divided by the light intensity in arbitrary units.
Figure 1. Photoionization threshold of CHI. The photoion yield is the ion counts at m/e 15 divided by the light intensity in arbitrary units. The threshold (at 125.9 nm, indicated by the arrow) was derived by applying a linear least-squares analysis to the linear ascending portion of the curve, 124-128 nm.
s
5*
1.41
1
OO
H2CN THRESHOLD REGION
-1
1.2c
a t BNL, N and F were generated in the same microwave discharge. In all experiments, [CH,] or [CD,] was in large excess ( - 5 X IOI3/cm3) compared to [F] (-lO1I/cm3) and the CH4 or CD4 was introduced through a moveable injector. The conditions were such that [N], was -5 X 1012/cm3and [CH,], was I011/cm3. Procedures for calibrating N, F, and CH3 have been discussed previously.'bs1c*12In these same references, sources of He, N2, NO, CF,, CH4,and CD4 are also given along with reagent purities and methods used for further purification.
-
Re!WltS The ionization thresholds for HzCN and D2CN were initially measured by using electron ionization in a discharge flow/mass spectrometer system. The signal at m / e 28 (H2CN+)or m / e 30 (D2CN+) was monitored as a function of electron energy (electronvolts). A plot was made of the ion signal vs electronvolts. Linear least-squares analysis was applied to the linear ascending portion of this curve. An apparent threshold energy was obtained by extrapolating to zero ion signal. To obtain the true ionization threshold, the mass spectrometer was calibrated by using HCN (IE= 13.6 eV) and N O (IE = 9.26 eV)." Using this calibration procedure, we obtained thresholds of 9.58 f 0.92 and 9.62f 0.58 eV for H2CN and DzCN, respectively. An average value of 9.6 f 1.0 eV was obtained, where the quoted uncertainty includes statistical errors at the 2a level plus an additional 15% for systematic errors. A preliminary report of this result has been given previously. I The electron impact results have large error bars due to the energy distribution of the bombarding electrons and the uncertainty in the calibration of the energy scale. Photons have a better defined energy and would allow a more accurate determination of the ionization threshold. The mass spectrometer was modified for photoionization. An argon resonance lamp (106.7nm, 11.6 eV) was used to detect H2CN and thus set an upper limit for its ionization threshold. The radical was readily detected by this method, thus demonstrating that the threshold for H2CN must be less than 11.6 eV. Photoionization spectra for H2CN and DzCN were obtained a t NSLS by use of the discharge flow/mass spectrometry technique coupled with dispersed synchrotron radiation as the ionizing agent. This is an excellent method for measuring ionization thresholds because of the well-defined and tunable vacuum ultraviolet photon source. The tunability feature of the light source allows for scanning through the threshold as well as above the threshold region to obtain the photoionization spectrum. As an example of the quality of data that can be obtained in the PIMS experiment, the photoionization spectrum of the methyl ~~
~~
(13) Lias, S.G.; Bartmess. J. E.; Liebman, J. F.; Holmes, J. L.;Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref D m 1968, 17, Suppl. No. I .
121
123
125
127
129
WAVELENGTH,
131
133
135
nm
Figure 3. Photoionivltion threshold region of H2CN. Apparent threshold is indicated by arrow 1; actual threshold is indicated by arrow 2 (see
text). H2 CN THRESHOLD i
1
I 130
I
132
134
136
WAVELENGTH. nm
Figure 4. Photoionization threshold of H,CN. The arrow indicates the threshold at 13 1.7 nm.
radical reactant is shown in Figure I . The indicated threshold at 125.9 f 0.3 nm corresponds to an ionization energy of 9.85 f 0.03 eV, which is in excellent agreement with the recommended value.I3 In the threshold region, the photoion signal to noise ratio is about 20:l and the threshold appears to be resolution limited (-0.3 nm fwhm). The small "tail" that extends toward longer wavelengths is also due to the slit function. Figures 2 and 5 show the photoionization spectra of H2CN and D2CN, respectively. The plots consist of the relative photoion signal (ion counts/light intensity in arbitrary units)" vs wavelength. The wavelength region covered was 1 l e 136 nm at 0.2-nm intervals. To obtain the ionization threshold, spectra were obtained in the narrow region 122-135 nm (Figures 3 and 6). Linear least-squares analysis was applied to the linear ascending portion of the curve. The straight line obtained was extrapolated to background photoion signal to obtain values for threshold energies. The apparent thresholds obtained for H2CN and D2CN, indicated by arrow 1 in Figures 3 and 6, are 9.69 f 0. IO and 9.72f 0.08 eV, respectively, thus giving an average value of 9.70f 0.10 eV.
The Journal of Physical Chemistry, Vol. 95, No. 20, I991 7615
Photoionization Spectra of H2CN and D2CN
order of relative energies (kilocalories per mole) of the indicated isomers of the radical: H2CN, 0 > H C N H (trans), 14 > H C N H (cis), 19 (4)
g
'I o
I
I
110
I
I
114
I
~
118
l
1
122
1
126
,
I
130
I
I
I
Thus, the ground-state isomer is H2CN. The process of formation via reaction 3 and the energetics summarized in (4) suggest that we are studying the H2CN isomer. Ab initio calculations1ss16may shed some light on the possible structures of isomers of the ion formed in these experiments. The more recent calcuIationsl6 suggest the following order of relative energies (kilocalories per mole) of the various singlet and triplet isomers of the ion:
I
134
singlet: HCNH+, 0 > HzNC+, 52
WAVELENGTH, nm
Figure 5. Photoionization spectrum of D,CN. The photoion yield is the ion counts at m / e 30 divided by the light intensity in arbitrary units. '
I
~
I
~
I
~
I
'
1.4
D2CN THRESHOLD REGION
0.8
1
9w > 2
Pn 121
123
125
127
129
131
133
135
WAVELENGTH, nm
Figure 6. Photoionization threshold region of D F N . Apparent threshold is indicated by arrow I ; actual threshold is indicated by arrow 2 (see
text).
Discussion and Conclusion The sources of neutral H2CN and its deuterated isomer D2CN in these experiments were the reactionsIb N
+ CH3
N
+ CD3
-
+H DzCN + D
HZCN
(3a)
(3b) Ab initio'& and transition-state theory'" calculations have been performed which yield the structures of the various isomers of neutral H2CN. The ab initio calculations1& suggest the following Bair, R.A.; Dunning, T. H., Jr. J . Chem. Phys. 1985.82, 2280. (b) Szekely, A.; Hanson, R. K.; Bowman, C. T. Inr. J. Chem. Kiner. 1983, 15. 91 5.
triplet: H2NC+, 1 1 3 > H2CN+, 120 I
(5) (6)
The ground-state structure of the ion is singlet HCNH', whereas ~ I singlet H2CN+ is a saddle point on the potential energy s u r f a ~ e . ' ~ ~ ' ~ The threshold value of 9.4 f 0.1 eV obtained for both H2CN and D2CN from the photoionization experiments is about 0.3 eV smaller than that obtained in the EIMS study. It is likely that this lower value was not observed in the electron impact work due to the very small cross sections involved here. The apparently small ionization cross sections could indicate vanishingly small Frank-Condon factors at threshold for the direct ionization process. In the absence of other information, e.g., photoelectron spectra and absorption spectra, the observed threshold therefore cannot be identified unambiguously with the ionization energy. On the other hand, it is important to point out that we believe the PIMS threshold a t 9.4 eV is free of perturbations due to hot bands because collisional deactivation of vibrationally excited H2CN radicals could be expected to occur with a time constant of about 1 ms under the present experimental conditions, assuming a deactivation efficiency of about 1 in lo3 collision^.^^ Computer simulations of the reaction system show that about 90% of the HzCN is formed within 4 ms of a total contact time of 5 ms. Thus, the bulk of the initially hot H2CN radicals formed via the N CH3 reaction would have sufficient time to be thermalized prior to being sampled. Further evidence to support the thermalization argument is the observation's that the ionization threshold for H2CN formed under similar conditions but via reaction of N atoms with C2HSis identical (9.4 eV) with that reported here even though the N C2HSreaction is 12 kcal/mol more exothermic than the N + CH3 reaction. Other effects related to photoelectron ionization or photoexcitation followed by photoionization may also be discounted as low-probability processes under the present conditions of low molecular beam density and a very low fluence, pulsed photon source (52 MHz). Before further discussion of the origin of the observed ionization threshold and spectra, it will be helpful to have estimates of several thermochemical quantities for H2CN and HCNH, especially the 298 K heats of formation ( A H I )of the radicals and radical ions since these lead directly to the ionization energy (IE). A H , (H2CN) has recently been estimated by Cowles et aI.l9on the basis of their determination of the electron affinity for HzCN. Their measured value EA (H2CN) = 11.8 kcal/mol leads to D (H2CNH) = 86 kcal/mol. From this, A H f (H) = 5 2 kcal/mol, and AHf (H2CNH) = 26 kcal/mol,20 they obtain A H f (HzCN) = 60 f 6 kcal/mol. This is in agreement with ab initio calculations2.1& and other estimates.Ia However, there has been a very recent determination of AHf (H2CNH) by Peerboom et aL2I which yields
+
Quoted errors are twice the standard deviations. The threshold region of the H2CN photoionization spectrum was examined in even closer detail, as shown in Figure 4. In this scan, a second threshold was obtained at 13 1.7 nm, yielding an ionization threshold of 9.4 f 0.1 eV. This is 0.3 eV lower than the apparent value quoted in the previous paragraph. The lower energy threshold is evident, however, in both the H2CN and D2CN spectra (arrow 2 in Figures 3 and 6), although it is somewhat obscured by a low signal to noise ratio (3:l). The data shown in Figure 4 were obtained under optimum conditions at the beginning of a fill on the VUV synchrotron ring. Under these conditions the signal to noise ratio was improved by about a factor of 2 compared to the data of Figure 3. On the basis of this result (Figure 4), we believe that the threshold indicated by arrow 2 (Figures 3 and 6) represents the actual threshold values for H2CN and D2CN. A prominent feature is observed in Figures 2 and 5 for HzCN and D2CN near 118.6 nm (10.5 eV). The significance of this feature as well as other less prominent ones will be examined further under Discussion and Conclusion.
(14) (a)
> H2CN+, 74
+
(15) Conrad, M. P.; Schaefer, H.F., 111. Narure 1978. 274,456. Allen, T. L.; Goddard, J. D.; Schaefer, H. F., H I . J. Chem. Phys. 1980, 73, 3255. (16) DeFrees, D. J.; McLean, A. D. J. Am. Chem. Soc. 1985, 107,4350. DeFrees, D. J.; Binkley, J. S.;Frisch, M.J.; McLean, A. D. J . Chem. Phys. 1986. 85, 5194. (17) Bradley, J. N . Shock Waoes in Chemistry and Physics; Wiley: New York, 1962. (18) Stief, L. J.; Nesbitt, F. L.; Tao, W.; Klemm, R. B. Unpublished
results.
(19) Cowles, D. C.; Travers, M.J.; Frueh, J. L.; Ellison, G. B. J. Chem. Phys. 1991, 94, 3517. (20) DeFrees, D. J.; Hehre, W. J. 1. Phys. Chem. 1978, 82. 391.
7616 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Nesbitt et al.
a considerably lower value (16.5 kcal mol) than measured by DeFrees and Hehre." Peerboom et a l l 1 also present arguments that the method employed previously20 may not be suitable for the determination of thermochemical parameters and that their lower value of AHf is more consistent with trends observed in the proton affinities of amines, imines, and nitriles. Using the newer value for AHf (H2CNH) (16.5 kcal/mol) yields from the data of Cowles et al.I9 a corrected AHf (H2CN) of 51 f 6 kcal/mol. This is still in reasonable agreement with calculated values that range from 55 to 59 k ~ a l / m o l . ~ ~ - ~ J ~ ~ As mentioned above, ab initio calculation^'^^'^ put H2CN+('Al) a t 73 f 4 kcal/mo122 above ground-state HCNH+ ( I F ) . AH, (HCNH') may be accurately determined from the proton affinity of HCN via the relationship AHAHCNH') = AHAHCN)
+ AHf(H+) - PA(HCN)
II 1I
01
(7)
Using AHf (HCN) = 32 f 2 k ~ a l / m o l , ' ~AHf . ~ ~(H+) = 366 f 0.01 kcal/mol,I3 and PA (HCN) = 171 f 2 kcal/mol,2'we obtain AHf (HCNH+) = 227 f 4 kcal/mol. Thus, AHf (H2CN+)= (227 f 4) + (73 f 4) = 300 f 8 kcal/mol at 298 K. It follows directly that IE (H2CN) = AHf (H2CN+)- AHf (H2CN) = (300 f 8) - (51 f 6) = (249 f 14) kcal/mol or IE (H2CN) = 10.8 f 0.6 eV. This is in good agreement with the value of 10.8 eV based on an open-shell CNDO c a l ~ u l a t i o n ,although ~~ this may be fortuitous given the very approximate nature of such calculations. Reference 25 contains a footnote indicating that the ionization energy for H2CN was measuted by Pottie and but this measurement was for H2CCN. A value for IE (HCNH) may be derived from the values given above for AHf (H2CN),the calculated energy separationIk between H2CN and HCNH (expression 4) and AHf (HCNH+). Thus, AH, (trans HCNH) = AHf (H2CN) A (H2CN-trans HCNH) = 51 14 = 65 kcal/mol. Similary, AHf (cis HCNH) = 51 + 19 = 70 kcal/mol. Since IE (HCNH) = AHf (HCNH+) - AHf (HCNH), we derive IE (trans HCNH) = 227 - 65 = 162 kcal/mol or 7.0 eV and IE (cis HCNH) = 227 - 70 = 157 kcal/mol or 6.8 eV. These values of 6.8 or 7.0 eV are much lower than the measured ionization threshold of 9.4 eV and the value derived above for IE (H2CN) = 10.8 eV. Also, these ionization energy values are consistent with our earlier conclusion, based on qualitative mechanistic and energetic considerations, that we have observed the H2CN isomer and not HCNH as the product of the reaction N CH3. We return now to consideration of the origin of the observed ionization threshold and the ionization spectra for H2CN and D2CN. The ionization threshold at 9.4 eV is 1.4 eV smaller than the estimated 10.8-eV ionization energy to the singlet H2CN conformation that was calculated to be a saddle point on the lowest ion hypersurface.IsJ6 The vertical transition cannot directly access the ionic H2CN conformation, and so the apparent vibronic spectral features suggest that the ionic HCNH conformation is accessed by autoionization from doublet Rydberg levels. Taking the ion limit at 10.8 eV, the first member of the Rydberg series should be at about 8 eV on the basis of the observation2' that this state lies roughly at 0.75 IE. The autoionizing Rydberg levels are sufficiently high that they can be supposed to behave like the limiting ion state. Large-amplitude motions of the hydrogen atoms are then easily excited, allowing access to the HCNH portion of the hypersurface. The neutral Rydberg states are above the ionization energy of this ground ion conformation, and autoion-
+
+
I
+
(21) Peerboom, R.A . L.; Ingemann, S.; Nibbering, N . M. M.; Liebman, J. F. J . Chem. Soc., Perkin Trans. 2 1990, 1825. (22) Uncertainty of f4 kcal/mol is based on private communication: H. F. Schaefer 111, 1990. (23) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, R. A,; McDonald, A,; Syverud, A. N . J . Phys. Chem. Ref Data 1985, 14, Suppl. No. I . (24) Tanaka, K.;Mackay, G. 1.; Bohme, D. K. Can. J . Chem. 1978, 56, 193. (25) Zahradnik, R.;Carsky, P. Theor. Chim. Acra 1972, 27, 121. (26) Pottie, R. F.; Lossing, F. P. J . Am. Chem. SOC.1961, 83, 4737. (27) Berkowitz, J . Phoroabsorption, Photoionization and Photoelectron Spectroscopy; Academic Press: New York, 1979.
I/ $1 HzCN 12Bzl
',",U:k',:;" 51
Figure 7. Energetics of neutral and ionic states of HzCN and HCNH discussed in the text. Values shown are AH,(kcal/mol) of each state.
ization can occur into vibrationally excited states. Apparently, the coupling into these states is sufficiently difficult for low members of the autoionizing Rydberg that the threshold occurs substantially above the n = 1 level at 8 eV. This might also explain the low ion count rate that was observed just above the threshold a t 9.4 eV. The energetics of the neutral and ionic states of H2CN and HCNH are summarized in Figure 7 along with the various calculated ionization energies and the observed autoionization via the H2CN doublet Rydberg state. T h e o r e t i ~ a l and ~ ~ ~experi~* menta12'3' studies of the electronic states of H2CN and D2CN have been reported. These states are highly p r e d i ~ s o c i a t e d , ~ ~ ~ ~ and the transitions are in the near UV (-280 nm). No transitions have been reported or predicted in the vacuum ultraviolet corresponding to the high-lying H2CN Rydberg state (8-10.8 eV or 155-1 15 nm). Thus, the features in the photoionization spectra may represent information (possibly vibrational structure) for the Rydberg state. They certainly provide a characteristic signature for the H2CN radial. To provide quantitative information on the H2CN Rydberg state, new experiments on the photoionization spectra at significantly higher wavelength resolution (perhaps 0.01 nm compared to the present 0.3 nm) and on the absorption spectra in the vacuum ultraviolet would be required. The various isomers of the H2CN radical and radical ions are believed to play a significant role in a vareity of phenomena. For example, in interstellar clouds, reactions of the isomeric ions HCNH+ and H2NC+ are considered to be important in the formation of interstellar HCN and HNC.8*'5*'6 The most frequently suggested mechanism is the reaction sequence C+ + NHj H2NC+ + H (8)
+
H,NC+ HCNH' (9) HCNH+ + e HCN (HNC) + H (10) The value for the ratio [HCN]/[HNC] is very large under conditions of thermodynamic equilibrium, but in molecular clouds a value near unity is typically observed." This equality of [HCN] (28) So, S.P. Chem. Phys. Lett. 1981. 82, 370. (29) Jacox, M. E.J . Phys. Chem. Re/. Data 1988, 17, 360. Jacox, M. E. 1.Phys. Chem. 1987, 91,6595. (30) Ogilvie, J. F. Chem. Commun. 1965, 359. Ogilvie, J. F.; Horne, D. G.J . Chem. Phys. 1968.48, 2248. Ogilvie. J. F. Can. J . Specrrosc. 1974, 19, 89. ( 3 1 ) Horne, D. G.;Norrish, R. G. W. Proc. R . SOC.London 1970, A315, 287. Horne, D. G.;Norrish, R . G. W. Proc. R. Soc. London 1970, ,4315, 301. (32) Dagdigian, P. J.; Anderson, W . R.; Sausa. R. C.; Miziolek, A . W. J . Phys. Chem. 1989, 93, 6059. (33) Wooten, A.; Evans, N . J.; Snell, R.; Vanden Bout, P. Astrophys. J . 1978, 225, L143.
J. Phys. Chem. 1991, 95,7617-7621 and [HNC] may be interpreted as evidence for their formation via ion-molecule chemistry involving HCNH+ and H2NC+ and the lack of isomerization of HNC to HCN at the very low densities prevailing in the interstellar medium. The results of this study have implications for the production of HCN on Titan. Current theories speculate that HCN can be p r o d u d on Titan by ion chemistry9 and neutral chemistry,' The ion chemistry involves the following series of reactions: N+ + CHI+ HCN' + H2 H (1 1) HCN" HCNH+
+ H2
+e
-
+ HCNH' + H H C N (HNC) + H
+
(12)
(10)
The neutral chemistry involves the following reactions:l N + CH3 HZCN + H +
H
+ H2CN
HCN
+ H2
(2) On the basis of the present study, we suggest that the neutral and +
7617
ion chemistry may be coupled in the upper atmosphere of Titan by the action of vacuum ultraviolet solar radiation on H2CN resulting in autoionization of H2CN to HCNH'.
Acknowledgment. We are grateful to Prof. G. Barney Ellison for helpful discussions and for communicating the data On the electron affinity and heat Of formation Of HzCN prior to publication; we thank Dr. William H. Kirchhoff for making us aware of the work of Prof. Ellison and his co-workers. We acknowledge helpful discussions with Drs. Sharon Lias, Marylin Jacox, and Joel Liebman, Prof. David Turner, and especially Dr. Morris Krauss. G.M. thanks the National Academy of Science for the award of a Research Associate. F.L.N. acknowledges support under NASA Grant NSG-5173 to the Catholic University of America. The work at GSFC was supported by the NASA Planetary Atmospheres Program. The work a t BNL was supported by the Division of Chemical Sciences, US.Department of Energy, Washington, DC, under Contract No. AC02-76CHOOO16.
Nature of Hydrogen Bonds Formed by Simple Amides and Sulfonic Acids in Inert Solvent. 1. 'H NMR Low-Temperature Studles Marek Ilczyszynt Dzpartement de chimie. Universite de Montreal, C.P. 6128, Succ. A, Montreal, Quebec, Canada H3C 357 (Received: December 27, 1990)
For the first time direct spectroscopic evidence was found that amides (B) are the 0 and N bases. *HNMR low-temptrature investigationsshow that sulfonic acids (AH) form with amides AH-O(B)N and AH-O(B)N-HA hydrogen-bonded complexes. The latter can exist in two different tautomeric states with exchange between them governed by the electron structure rearrangement of the complexed amide. The O H 0 proton in the AH-O(B)N complexes probably occupies only one position inside the hydrogen bridge (one tautomeric state of the complex) which is sensitive to the acid-base interaction strength and to the solvation. These phenomena are monitored by its chemical shifts.
Introduction Hydrogen-bonded complexes formed by trifluoromethanesulfonic acid (CF3S03H), methanesulfonic acid (CH3S03H),and ptoluenesulfonic acid (C7H,S03H) with NJVdimethylformamide (DMF) and NJV-dimethylacetamide (DMA) in dichloromethane solutions were investigated. CF3S03H is known for its interesting properties,' great ability to hydrogen bond with water,2 and self-a~sociation.~It is one of the strongest acids and is frequently used in different protonation reactions.c8 To my knowledge there is only one paper2 dealing with its behavior as the acid in an aprotic and inert solvent. D M F and DMA are usually treated as the oxygen bases, but the nitrogen is sometimes taken into account as the additional basic center.*I2 Evidences for the N-protonation are drawn mainly from kinetic investigation^'^-'^ and are supported by quantum mechanical calculations.I6 S ectroscopic results for crystal^'^ and strongly acidic solutions1g-20prove the interactions to the oxygen atom only whereas the results obtained for the less active media are controversial or provide indirect evidences for the N
interaction^.'^+^*^^ Experimental Section
CF3S03H (Aldrich), CH3S03H (Fluka), D M F (ACP), and DMA (Aldrich) were distilled under reduced pressure in a dry nitrogen atmosphere. C7H7S03Hmonohydrate (Aldrich) was dried under vacuum at 50 OC for 190 h; the final pressure was 'Present address: Institute of Chemistry, University of Wroclaw, ul Joliot-Curie 14. 50-383Wroclaw, Poland.
