602
J. Phys. Chem. 1982, 86, 602-606
model. Table I11 is the correlation matrix for the model. It should be emphasized that the structure represented by model D4 is virtually independent of the amplitude assumptions contained in it; simultaneous refinement of all amplitudes and geometrical parameters with rotational constant weighting as in D4 led to maximum changes of 0.001 A in any distance and 0.1' in any bond angle. The amplitude values themselves with estimated uncertainties encompass those listed in Table I1 for model D4. These amplitude values may be seen in Table IX of the preceding article. Summary and Discussion It will be recalled that the earlier investigation of the SOF, structure in this laboratory led to the discovery of four models giving equally good fits to the diffraction data. The principal result of this reinvestigation has been the identification of a single model, virtually identical with one of the four from the first study, which we now adopt as the best representation of the structure. This identification was made possible in large measure by including the three rotational constants in the refinement procedure. These data, which were unavailable at the time of the earlier work, serve to eliminate unequivocally two of the four models found to be acceptable judged on the diffraction data alone. Each of the remaining models (types B and D in the early work) gives an excellent fit to both sets of data. Our final choice of the latter type, D4, as the "best model" rests on less unequivocal, but still entirely convincing evidence: the amplitude l(F,.O) at 0.043 (4) 8,for the B-type model is implausibly small and the amplitude 1(F,.F ) at 0.093 (7) A implausibly large. It must be admitte3 that this amplitude evidence against model B was present in the early work where it was disregarded in favor of some speculative arguments, weak or nonexistent in retrospect, which led to model B being slightly favored over the others. Our present judgement is that the amplitude evidence is compelling, and that its rejection in the early work reflects what we now see as an unwarranted skep-
ticism about the reliability of amplitude measurements. The evidence on this point is found partly in our accumulation of general experience. It is also found in comparison of the amplitude results for SOF, from this and the earlier investigation; the values are very similar despite the use of computer-generated backgrounds in one case and hand-drawn ones in the other for extraction of the molecular intensities. Our best model for SOF, has parameter values, both structural and vibrational, virtually identical with those obtained for model D in the earlier investigation. As was pointed out there, the bond lengths are about the same as in sulfuryl fluoride', and the S-F bond length corresponds to considerable double-bond character. The bond angles are in accord with expectation from VSEPR theory; the S=O double bond occupies more space that the single bonds leading to an F,,SO angle larger than F,,SF,, and a S-F, bond longer than S-F,,. The comparison between our experimental results and those calculated by Oberhammer and Boggs using ab initio methods with full geometry optimization5is worth noting. Their results followed in each case by our r," values (which are most nearly comparable to their re ones) are r(S=O) = 1.401 A, 1.408 (4) A; r(S-F ), = 1.540 A, 1.538 (3) A; r(S-F,) = 1.579 A, 1.595 (3) A; LF, SF,, = 162.4', 164.6 (6)'; LF,,SF, = 113.3O, 112.8 (4)'. %h'e agreement is impressive.
Acknowledgment. We are grateful to the National Science Foundation for support of this work under grants CHE 78-04258 and CHE 81-10541. We thank Professor Graybeal for communicating his values for the SOF, rotational constants and Dr. Czerepinski for his helpful comments about sample purity. Supplementary Material Available: Tables of total scattered intensities, calculated backgrounds, and average molecular intensities from the two camera distances (7 pages). Ordering information is found on any current masthead page.
Molecular Structure of Hydrazine As Studied by Gas Electron Diffraction Kunlo Kohata,t Tsutomu Fukuyama,t and Koro Kuchltsu
+
Department of C h e m l s ~faculty , of Sclence, The Unlverslty of Tokyo, Bunkyeku, Tokyo 113, Japan (Received: July 24, 198 1; In Final form: September 18, 1981)
The geometrical structure of hydrazine has been determined by an analysis of the electron diffraction intensity and the rotational constants for the normal species reported by Kasuya by microwave spectroscopy: r,(N-N) = 1.449 f 0.002 A, rg(N-H)av= 1.021 f 0.002 A, LN-N-H, (outer) = 106 f 2', LN-N-H~(inner) = 112 f 2 O , dihedral angle = 91 2 O , where uncertainties represent estimated limits of the experimental error. The inner and outer N-N-H bond angles are found to be significantly different.
*
Introduction The structure of hydrazine (Figure 1)in the gas phase has been studied by electron diffraction,' infrared spectroscopy,24 microwave spectroscopy,58 and photoelectron spectros~opy.~The N-N and N-H bond lengths determined by electron diffraction1have been quoted frequently in discussions of the structure of this molecule and related 'National Institute for Environmental Studies, Tsukuba, Ibaraki 305, Japan. 0022-3654182f 2006-0602$0 1.2510
molecules. They were also used in analyses of the rotational constants derived from spectroscopyz-8to determine ~
~~
(1)Y.Morino, T. Iijima, and Y. Murata, Bull. Chem. SOC.Jpn., 33, 46 (1960). (2)Y.Yamaguchi, T. Ichishima, T. Shimanouchi, and S. Mizushima, Spectrochrm. Acta, 16, 1471 (1960). (3) Y. Hamada, A. Y. Hirakawa, K. Tamagake, and M. Tsuboi, J. Mol. Spectrosc., 35, 420 (1970). (4)M. Tsuboi and J. Overend, J . Mol. Spectrosc., 52, 256 (1974). ( 5 ) T. Kasuya, Sci. Pup. Znst. Phys. Chem. Res., 56, 1 (1962). (6)T. Kasuya and T. Kojima, J . Phys. SOC.Jpn., 18, 364 (1963).
0 1982 American Chemical Society
Molecular Structure of Hydrazine
The Journal of Physical Chemistry, Vol. 86, No.
5, 1982 603
L
Hi2
(0) (b) Figure 1. Hydrazine (a) as viewed along the C 2 axis and (b) as viewed a m the N-N axis from the N, atom. The hydrogen atoms whlch take the inner and outer positions are denoted as i and 0, respectively. The H-N-N-H dihedral angle, 8, is deflned as the angle between the two bisectors of the H-N-H bond angles.
the positions of hydrogen atoms. In particular, the dihedral angle, 0, was estimated to be about 90°, and the potential function for the internal rotation about the N-N axis has been studied with the aid of theoretical calculations.l0J1 However, none of the previous structure determinations seems to be complete for various technical reasons: (1) The electron diffraction study of Morino et al.' was made in the relatively early stages of the sector-microphotometer method. Today one can measure the electron diffraction intensity with much higher accuracy. In addition, one can estimate the parameters related to hydrogen atoms by a combined analysis of the diffraction intensity with the rotational constants, a technique that has since been developed.l*l4 (2) The spectroscopic structures determined only from the rotational constants are liable to considerable systematic errors resulting from various as~umptions.'~J~ Though the rotational constants for several isotopic species (such as dz and d4) are, in principle, usable for structure determination,7s8they may easily become sources of systematic errors because it is hard to make accurate estimates of the isotopic dependences in the vibrationally averaged geometrical parameters. The average nuclear positions, particularly those of hydrogen atoms, determined from the isotopic differences of the rotational constants are known to depend very sensitively on how these isotopic dependences are assumed,14and there seems to be no experimental or theoretical way to estimate them accurately. Therefore, the use of isotopic rotational constants does not necessarily contribute to the accuracy of the parameters determined, particularly those involving hydrogen atoms. For hydrazine, at least five isotopic dependences have to be estimated accurately in order to take the three rotational constants for ND2NDzinto the analysis of the vibrational average structure, and hence, this method seems to be hardly practicable at the present stage.' The situations are even worse for mixed H-D isotopic species. Under these circumstances, it was thought worthwhile to reinvestigate the geometrical structure of hydrazine, which is one of the simplest inorganic molecules of fundamental importance. The electron diffraction intensity was obtained with an improved microphotometer system, and the rotational constants for the normal species reported by Kasuya5 was taken into a combined analysis; (7) S. Tsunekawa, J. Phys. SOC. Jpn., 41,2077 (1976). (8)S.Tsunekawa and T. Kojima, J. Phys. SOC.Jpn., 44,1925 (1978). (9)K. O d u n e , S.Kataumata, and K. Kimura, Chem. Phys. Lett., 19, 369 (1973). (10)L.Pedersen and K. Morokuma, J. Chem. Phys., 46,3941 (1967). (11)P. B. Ryan and H. D. Todd, J. Chem. Phys., 67, 4787 (1977). (12)K. Kuchitau, =FiftyYeam of Electron Diffraction",P. Goodman, Ed., International Union of Crystallography, Reidel, Dordrecht, 1981, Part 111, Chapter 3. (13)K. Kuchitau and K. Oyanagi, Faraday Discuss. Chem. SOC.,62, 20 (1977). (14)M. Nakata, K. Kohata, T. Fukuyama, and K. Kuchitsu, J. Mol. Spectrosc., 83, 105 (1980).
Figure 2. Molecular intensities for hydrazine. Observed values are shown as open circles, and the solid curve represents the best-fit theoretical intenslties. The lower plot represents the residuals.
1 0
N-H
1.0
2.0
3.0
(A)
Figure 3. Experimental and theoretical radial distribution curves. A damping factor, exp(-0.0016s2), was used. The residuals are shown below on the same scale.
other isotopic rotational constants were not used for the reason described above. The result of a recent ab initio ~alculation'~ was also taken into the analysis in order to estimate the parameters of minor importance which could not be determined in the analysis. A particular effort was made to examine, for the first time, whether the two inequivalent N-N-H angles, LN-N-Hi and LN-N-H, shown in Figure 1, were significantly different. The diffractionspectroscopy joint analysis showed that it was indeed the case. Experimental Section Hydrazine hydrate (purchased from Tokyo Kasei Co., Ltd.) was refluxed with an equimolar proportion of sodium hydroxide for 6 h at about 100 "C under argon atmosphere of 10 torr, and the product was further dehydrated with barium oxide in a vessel attached to a vacuum line. The sample was estimated to be more than 99.9% pure from its mass spectrum. No decomposition was observed in the glass vessel during the experiment. Diffraction photographs were taken at camera distances of 113.1 and 247.1 mrn with an apparatus equipped with an r3 sector.16 The accelerating voltage of the electrons, about 40 kV, was stabilized within f0.01% during the experiment. The nozzle tip was made of glass to avoid sample decomposition on the surface, coated outside with gold to assure electric conduction, and used with hightemperature equipmental' The sample reservoir, the supply line, and the nozzle were heated to 60 "C;at this temperature the pressure of the sample was about 90 torr. The scale factors for the diffraction patterns were cali(15)Y. Hamada, N.Tanaka, and M. Tsuboi, private communication. The calculation was made with basis set 6-31G* with CI. (The asterisk means that a polarization function (a set of d orbitals) was included.) (16)Y. Murata, K.Kuchitau, and M. Kimura, Jpn. J. Appl. Phys., 9, 591 (1970). (17)A. Yokozeki and K. Kuchitau, Bull. Chem. SOC. Jpn., 44, 72 (1971).
604
Kohata et al.
The Journal of Physical Chemistty, Vol. 86,No. 5, 7982
TABLE I: Vibrational Frequencies for Hydrazine (cm- ) sym species A
sym species B
calcda
obsdb
calcd
obsd
3348 3242 1586 1280 1086 835 372
3334 3274 1579 1284 1035 896 371
3335 3232 1579 1262 1023
3336 3293 1598 1285 914
a Frequencies calculated by t h e use of the force constants given in ref 1 5 ( b u t after slight modifications, see text). Observed by infrared spectroscopy, ref 23.
brated to within 0.10% with reference to the r,(C=O) distance of carbon dioxide16measured under essentially the same experimental conditions. Other experimental details are described in ref 16 and 18. Two photographic plates taken at the short camera distance and three plates taken at the long distance were used for the intensity An outline of our measurement by a microphot~meter.~~ photometer system is described in Appendix. After correction for imperfection in the sector shape and drawing smooth background curves with spline functions, molecular scattering intensities were obtained in the ranges s = 3.5-18.8 A-1 and s = 7.9-34.6 A-' from the long and short distance data, respectively, at intervals of As = 7/10. Since the intensities from the two camera distances agreed in their overlapping region, they were joined at s = 12.6 A-l. The elastic and inelastic scattering factors and the phase shifts used for the correction for nonnuclear scattering were taken from the tables prepared by Schafer et alam The molecular intensity and the corresponding radial distribution curve are shown in Figures 2 and 3. Most of the calculations were carried out on a HITAC 8800/8700 computer in the Computer Centre of the University of Tokyo. Analysis of Electron Diffraction Data Since the molecule has Cz symmetry, as shown in Figure 1, seven parameters are necessary to define the molecular geometry: two N-H bond lengths, the N-N bond length, two N-N-H angles, the H-N-H angle, and the dihedral angle. The diffraction intensity was found to be sensitive to r(N-N), average r(N-H), and average N-N-H angle; in addition, a significant difference between the N-N-Hi and N-N-H, angles was indicated. The influences of the difference between the N-Hi and N-H, distances and the H-N-H angle were found to be insignificant and have hardly any effect on the accuracy of the rest of the parameters; hence they were assumed to be 0.003 A and 106.6', respectively, as derived from the recent ab initio calculation.15 The dihedral angle was assumed at this stage to be 90.03°;5 however, it was possible to release this constraint at a later stage. "he mean amplitudes and the vibrational correctionsZ1z2 for the shrinkage effects, r, - ra, were calculated for all the internuclear distances on the basis of a theoretical force field. The force constants given by an ab initio calculation15 were here multiplied by 0.865 so as to improve the fitting between the calculated and observedz3vibrational (18) M. Tanimoto, K. Kuchitsu, and Y. Morino, Bull. Chem. SOC.Jpn., 43. (1970). --,-2776 ~----, (19) Y. Morino, K. Kuchitau, and T. Fukuyama, Bull. Chem. SOC.
Jpn., 40, 423 (1967). (20) L.Schhfer, A. C . Yates, and R. A. Bonham, J. Chem. Phys., 55, 3055 (1971). (21) K. Kuchitau and S. Konaka, J . Chem. Phys., 45, 4342 (1966). (22) K. Kuchitsu and S. J. Cyvin, "Molecular Structures and Vibrations", S. J. Cyvin, Ed., Elsevier, Amsterdam, 1972, Chapter 12.
TABLE 11: Mean Amplitudes ( Z i j ) and Shrinkage Corrections (Pa - r a ) for Hydrazine (in A)a 1;; r, - rrv N,-Hii Nl-H, N , -N, Ni-Hiz N l -Hoz Hi1 -Ho 1 Hil -Hi, Hi1-Hi* H01-Hiz
743 737 512 1004 1025 1198 1704 1685 1258
130 127 -13 15 24 131 28 - 23 0
a Mean amplitudes calculated for 6 0 "C. See Figure 1 for the numbering of the atoms, See ref 22 for the definition of r, and re.
TABLE 111: Observed and Calculated Rotational Constants for Hydrazine" (in cm-' ) A
B C
4.7855 ( 1 ) 0.80337 ( 4 ) 0.80294 ( 4 )
4.803 ( 2 ) 0.8014 ( 2 ) 0.8010 ( 2 )
4.80 ( 3 ) 0.801 ( 3 ) 0.800 ( 3 )
4.803 ( 3 ) 0.8015 (1) 0.8010 (1)
Uncertainties attached t o t h e last significant digits are Observed rotational constants for given in parentheses. the ground vibrational state, ref 5. A , has been recalcuZero-point average rotational constants lated (see text). calculated from A , , B o , and C, with corrections for vibrational effects. Reference 21. Rotational constants calculated from the polo parameters, which are compatible with the structure determined in the analysis of electron diffraction intensities. Uncertainties are estimated from those in the role parameters. e Best-fit rotational constants derived from a combined analysis of diffraction and microwave data. Uncertainties represent 2.5 times the estimated standard deviations.
frequencies. The frequencies are compared in Table I, and the calculated mean amplitudes and shrinkage corrections are listed in Table 11; their uncertainties are estimated to be 10%. All the mean amplitudes except for N-N and N-H were held constant at the calculated values. The phase modulation parameter22.24for the N-H bond was A3 from the anharmonicity estimated to be 1.1 X parameter, a3,for N-H diatomic molecule.25 The phase modulation parameters for the other atom pairs were ignored. The ra structurez2 was determined by a leastsquares analysis of the molecular intensities. A diagonal weight matrixz6was used in the least-squares fitting. The above analysis gave two inequivalent nonbonded Ne-H distances which differed by about 0.06 A. However, it was impossible to assign them to the inner and outer hydrogen atoms, because such an assignment required a precise measurement of the nonbonded H-H distances, on which the diffraction intensity contained little information. Nevertheless, it was possible to make an unambiguous assignment by an analysis of the rotational constants, as described in the next section. Joint Analysis of Diffraction Data and Rotational Constants The rotational constants for the ground vibrational state, Ao, Bo, and Co, were reported by Kasuya for the normal specie^.^ After recalculation of A. from the six transitions (23) A. Yamaguchi, Nippon Kagaku Zasshi, 80, 1109 (1959). (24) The phase modulation parameter, K , which appears in the expression of molecular intensity of electron diffraction, sin s(r, - KS?, is
often called the asymmetry parameter. It should not be confused with Ray's asymmetry parameter, which is also denoted usually as K , used in the present analysis of microwave rotational constants. (25) K. Kuchitsu, Bull. Chem. SOC.Jpn., 40, 505 (1967). (26) Y. Morino, K. Kuchitau,and Y. Murata, Acta Crystallogr., 18,549 (1965).
The Journal of Physical Chemistry, Vol. 86,No. 5, 1982 605
Molecular Structure of Hydrazine
TABLE IV: Comparison o f Rotational Constants and Asymmetry Parameterf
A B
C K~
Zb
Dloc
a 0 C
4.803 ( 2 ) 0.8014 (2) 0.8010 (2) -0.9998 ( 4 )
4.80 (3) 0.801 (3) 0.800 ( 3 ) -0.9996 (5)
4.80 (3) 0.806 ( 3 ) 0.800 ( 3 ) -0.9969 (5)
106 112
LN-N-Ho L N-N-Hi
112 106
Uncertainties attached to t h e last significant digits are given in parentheses. Rotational constants are in cm'' units. b Zero-point average rotational constants calculated from A , , Bo,and C, (see Table I11 with corrections for Rotational convibrational effects). Reference 22. stants calculated from t h e r, parameters determined in the analysis of electron diffraction intensity with t w o alternaRay's asymmetive assignments o f the N-N-H angles. try parameter defined as K = (2B - A - C ) / ( A- C). TABLE V: Correlation Matrix X 100 for Hydrazinea
k, kZ X l
XZ
x3 x4 x5
4 4
k, 100
k, 43 100
x2 x3 x4 -4 11 36 -37 33 -36 -25 18 100 -95 - 3 0 5 100 47 -24 100 -97 100 XI
x5 I1 4 -37 55 50 22 72 89 19 21 26 -36 -21 -27 -99 - 5 -16 99 - 0 10 100 3 14 100 69 100
k , , k , are indices of resolution for the long and short camera distance data, 1.00 (3) and 1.03 (6), respectively. x , = r(N-N), x 2 = r(N-H), x 3 = LN-N-H,, x , = LN-N-Ho, x , = dihedral angle, I , = mean amplitude (N-H), I , = mean amplitude (N-N).
reported by Kasuya,2' these constants were converted to the zero-point average rotational constants, A,,B,, and C,, by harmonic correction^,^^^^^, as listed in Table 111. The uncertainties in these corrections were estimated to be lo%,which constituted a major part of the uncertainties in the average rotational constants. The r, structure determined by electron diffraction was extrapolated to 0 K by estimating the difference, r, - r,o.22 In order to assign the two N-N-H angles the rotational constants and Ray's asymmetry parameter,29K = (2B - A - C)/(A- C), calculated from the r 2 structures, were compared with the corresponding observed values in Table IV. The K parameter was found to depend sensitively on the assignment of the angles. It was concluded that the inner N-N-H angle was larger than the outer angle. This assignment resulted in average rotational constants, A:, B2, and C2,in agreement with A,,B,, and C, within the experimental uncertainties of the former, as shown in Table 111. The latter constants and the diffraction intensities were thus taken as observables in the least-squares analysis. The statistical weights for the rotational constants were estimatedm to be 2.2 X lo4, 7.7 X lo5,and 6.8 X lo5for A,,B,, and C,, respectively, while a unit weight was assigned to the molecular intensities from s = 10.5 to 25.1 A-l. It was now possible to take the dihedral angle as an additional variable parameter. The correlation (27) Kasuya's eq 4 in ref 5 should read as follows: W(JJ0 = ( A + B)J(J + 1)/2 + [C- ( A + B ) / 2 ] I P- D g ( J + 1)' - DjKJ(J + 1)IP -
D&'.
The recalculation was based on this revised equation. (28) M. Toyama, T. Oka, and Y. Morino, J. Mol. Spectrosc., 13, 193 (1964). (29) W. Gordy and R. L. Cook, 'Technique of Organic Chemistry", Vol. IX, Part 11, W. West, Ed.,Wiley, New York, 1970, Chapter 7. (30) K. Oyanagi and K. Kuchitau, Bull. Chem. Soc. Jpn., 51, 2237 (1978).
TABLE VI: Molecular Structure and Mean Amplitudes for Hydrazine Determined from Electron Diffraction and Microwave Datau b
re b 1.021 (3) 1.449 ( 2 )
rav 1.015 ( 2 ) 1.447 ( 2 ) 106 ( 2 ) 112 ( 2 ) 91 (2) 106.6 (assumed) 0.058 ( 3 ) 0.080 ( 3 )
r(N-H)" r(N-N) LN-N-Ho LN-N-Hi ed
LH-N-H~ z(N-N)~ I(N-H)~
Bond distances are in A units. See ref 22 for the definition of ray and r . Angles are in degrees. Estimated limits of error are listed in parentheses. Average of r(N-Hi) and r(N-Ho), It was assumed that r(N-Hi) r(N-Ho) = 0.003 A . The dihedral angle defined as that between t h e t w o bisectors of t h e H-N-H angles. See Figure 1. e Assumed to the value obtained b a n a b initio calculation. Reference 15. (See text.) Observed mean amplitudes.
-
Y
TABLE VII: Comparison of Structural Parameters of Hydrazineu ED t
MWb
E DC
MWd
IRe
1.021 ( 3 ) 1.022 ( 6 ) 1.008 ( 8 4 1.016 1.449 ( 2 ) 1.449 ( 4 ) 1.447 ( 5 ) 1.446 106.6' 1 1 3 (.3 ,) 106 (2) 112.0 (15) 109.2 ( 8 ) 108.85 112 ( 2 ) 88.9 (15) 88.05 8' 91 ( 2 ) Uncertainties attached t o t h e last significant digits are given in parentheses. Bond distances are in A units. Angles are in degrees. Present study. Bond distances are listed as rg, and bond angles are calculated o n t h e basis of t h e r, structure. Reference 1. d Reference 7. The structural parameters were determined so as t o reproduce the observed rotational constants for four isotopic species (normal, t w o 1,2-dz,and d4) by a least-squares method. e Reference 4 . Estimated from t h e rotational constants, A - ( B t C)/2 and ( B t C)/2, as a possible and probable set with n o explicit estimates of uncertainty. An effective N-H distance, ro(N-H), which is expected t o be about 0.01 A shorter than rg(N-H). g Assumed value based o n an a b initio calculation. Reference 15. The outer (0)and inner (i) N-N-H bond angles. Dihedral angle. r(N-H) r(N-N) LH-N-H LN-N-H LN-N-H?
TABLE VIII: Comparison of Structural Parametere
r(N-H) LX-N-Hf
NH,NH,, rav 1.015 ( 2 ) (2)g 112 (2)h
ro 1.016 (8) 103.3 ( 5 )
CH,NH,,C NH,,d ro re 1.0096e 1.0116e 110.3 106.68
a Uncertainties attached t o the last significant digits are given in parentheses. Bond distances are in A units, and angles are in degrees. Note that the slight differences in the definitions of the structures given (ref 22) are not essential for this comparison. Reference 32. ReferReference 34. e Uncertainties are not listed ence 33. in t h e original reference. f X = N 0, C, and H atoms. g The outer N-N-H bond angle. The inner N-N-H bond angle.
matrix is shown in Table V. The rav structurez2derived from this analysis agreed with the r,O structure; it is listed in Table VI. The N-N and N-H mean amplitudes also converged to essentially the same values as those obtained in the analysis of the diffraction data alone. They are slightly larger than the values calculated from the force constants. In fear of the existence of small systematic errors in the intensity measurement, the influence of the fixed parameters, particularly the mean amplitudes of the
J. Phys. Chem. 1982, 86, 606-612
808
N-0.H nonbonded pairs, was examined, and the uncertainties of this origin were included in the error estimates. The limits of uncertainty in the structural parameters listed in Table VI were based on our estimates of random and systematic errors.31 Comparison of Structural Parameters The structural parameters obtained in the present study are compared in Tables VI1 and VI11 with those reported previously for hydrazine and related molecule^.^^-^^ It turns out that the N-N and N-H bond lengths reported by Morino et al.’ are remarkably accurate. The dihedral angle, 9l0, is also compatible with the values determined by microwave and infrared studies.2-8 The presence of a significant difference between the inner and outer N-N-H bond angles has now been demonstrated experimentally. The same trend has also been predicted by a recent ab initio ~alculation.’~ The outer N-N-H angle is similar to the H-N-H angle of ammonia,34whereas the inner angle is about 5’ larger, The structure of hydrazine determined in the present study is still far from complete. Even a combined analysis of electron diffraction and high-resolution spectroscopy has been able to solve the problems described in Introduction only partly. One of the future steps toward our goal seems to be to combine ab initio calculations, particularly in making even a crude set of estimates of the isotopic differences in the vibrationally averaged nuclear positions on the basis of the cubic force field, so that one can take all the available isotopic rotational constants into the analysis.
Acknowledgment. The authors are grateful to Professor M. Tsuboi and Dr. Y. Hamada for providing data on the force field and the structural parameters derived from their (31)K. Kuchitau, ‘Molecular Structures and Vibrations”, S. 3. Cyvin, Ed., Elsevier, Amsterdam, 1972,Chapter 10. (32)S.Teunekawa, J.Phys. SOC. Jpn., 33, 167 (1972). (33)K. Takagi and T. Kojima, J. Phys. SOC.Jpn., 30, 1145 (1971). (34)W.S.Benedict and E. K. Plyler, Can. J.Phys., 35,1235(1957).
calculations prior to publication. Appendix. Microphotometry The electron diffraction intensity recorded on a photographic plate is measured by a microphotometer (Rigaku Denki MP3).19 The plate is spun by a synchronous motor during the photometry at precisely 150 rpm in order to smooth out fluctuations due to unevenness of the emulsion, and it is driven at a constant speed of 2 mm/min along the diameter of the halos. The sampling is carried out at 1-8 intervals for 400 ms. A pair of 6-V batteries (102 A h for each) connected parallel is used for the light source (a dc lamp). Random fluctuations of the voltage from linearity are less than 0.00190 and the gradient, about 2 pV/s, is compensated for by subtraction of a linear function. The electric current from a photomultiplier, about 0.1 PA, is grounded through a high-precision resistor of 2.5 MQ. The output voltage is integrated to eliminate random noises in the photocurrent for 400 ms, during which time the plate makes exactly one full turn. The voltage measured to six decimals is read from a digital voltmeter as parallel binary signals. They are serialized by an interface and fed into a microcomputer (NEC, TK85). In our previous measurements of photographic densities the sampling interval was set to As = 7/10 A-l. About 100 data were obtained for each photographic plate taken with a camera distance of 107 mm. In the present scheme more than 3000 data can be measured with intervals of about 0.01 A-l, and about 600 data obtained by averaging are used for subsequent analyses. Several test measurements on the same photographic plates have shown that the random standard error in the molecular intensity has been reduced to about one-half of that in our previous measurements.
Supplementary Material Available: Experimental data of the tobl intensity for hydrazine obtained by gas electron diffraction (2 pages). Ordering information is given on any current masthead page.
Tlme-Dependent Mass Spectra and Breakdown Graphs. 2. The Klnetlc Shift in Pyridine Chava Llfshltz L?Spartment of Physical Chemkhy, The Hebrew Universtty of Jerusalem, Jerusalem 91904, Israel (Received June 12, 1981; I n Nnal Form: October 3, 1981)
-
Appearance potentials, breakdown curves, and metastable peak shapes were determined as a function of time, by trapped ion mas8 spectrometry (TIMS), for the reaction C6H5N+ C4H4++ HCN, in pyridine. The experimental data were compared with QET calculations of time-resolved appearance potentials, breakdown curves, and crossover shifts. Good agreement was obtained between the present electron impact data and previous photoionizationand photoion-photoelectron coincidence data. The long-time (milliseconds) appearance potential value limit is AP(C4H4+)= 11.95 f 0.2 eV at 423 K. Introduction Two factors causing appreciable errors in obtaining thermochemical data from appearance potential measurements have existed since the early days of mass spectrometry:l (1)the swalled “kinetic shift”,the excess (1)W. A. Chupka, J. Chem. Phys., 30,191 (1969);L. Friedman, F. A. Long, and M. Wolfsberg, ibid., 26,714 (1957). 0022-3654/82/2086-0606$0 1.25/0
energy required to produce detectable dissociation in S, and (2) the temperature of the gas in the ionbiW region. The introduction2Of SUperSOniC beinto photoionization m a SpedrometW has bled the Cooling of gases to VerY (2)G.R. Parr and J. W. Taylor, Rev. Sci. Instrum., 44,1578(1973); C.Y.Ng, B. H. Mahan, and Y. T. Lee, J.Chem. Phys., 65,1956(1976), W. M.Trott, N. C. Blais, and E. A. Walters, ibid., 69,3150 (1978).
0 1982 American Chemical Society
