1915
CRYSTAL STRUCTURE OF PYRIDINE HYDROGEN NITRATE
The Crystal Structure of Pyridine Hydrogen Nitrate'
by Aubrey J. Serewicz, B. Ken Robertson, and Edward A. Meyers Department of Chemistry, Texas A & M University, College Station, Texas
(Received December 14, 1964)
Pyridine hydrogen nitrate is moonoclinic, space group P21/c, with cell parameters a = 3.905, b = 12.286, c = 13.470 A., 0 = 90" 30' and four molecules per unit cell. The structure was determined by three-dimensional X-ray analysis, and the final parameters were refined with anisotropic temperature factors by least-square analysis of hlcl (h = 0, 1, 2) data ( R = 0.085). Correction for thermal motion gives the following average bond distances: N(l)-C(l) = 1.375, N(l)-C(5) = 1.333, C(l)-C(2) = 1.383, C(2)-C(3) = 1.367, N(2)-O(1) = 1.291, N(2)-O(2) = 1.266 A. A comparison has been made between pyridine hydrogen nitrate and molecules of similar structure.
Introduction While attempting to prepare single crystals of the positive halogen compound, IPyJJOa,2t it was found that PyHNO, was produced as the major product of recrystallization from ether-chloroform mixtures, presumably after reaction of the original compound with the solvent. The crystals were initially clear and colorless but after exposure to light became yellowish and at times showed evidence of a dark material deposited in the center of the whiskers. An analysis of the compound showed about 6.801, of iodine in some way present in the recrystallized material. It was decided that a structure determination of pyridine hydrogen nitrate would be useful for several reasons. First, it would serve as a preliminary to further studies of the iodinecontaining materiaI. Second, it would be of interest in itself to examine the structure of the pyridinium and nitrate groups without disturbing effects due to heavy atoms. Third, the preliminary examination of the material revealed that a very easy twisting distortion of the crystals around the needle axis was accompanied by a very short axial length along the needle axis, which indicated some sort of layer structure in this presumably ionic material.
Experimental Space Group and Unit Cell Dimensions. Pyridine hydrogen nitrate was first synthesized by Anderson (1858) and given a more complete characterization by Pincussohn (1897). It is a 1: 1 compound formed by mixing equimolar amounts of concentrated nitric acid and dried pyridine. It crystallizes readily from solution
a t -78" into long, translucent, and easily deformable crystals. The crystals require considerable care in handling to minimize deformation. Examination of single crystals of the compound with a Buerger precession camera ( N o IG radiation) revealed systematic absences for (h01) when 1 was an odd integer and for (OkO) when k was an odd integer, characteristic of P21/c-C2h5. The unit cell dimensions are a = 3.905, b = 12.286, c = 13.470 8.)and = 90' 30'. The density observed by the flotation method is 1.432 f 0.020 g./cc.; the density calculated for four molecules per unit cell is 1.4.54 g./cc. Measurement and Correction of Intensities. A needle approximately 0.3 nim. in diameter (crystal I) was mounted for rotation around the a-axis, and 30' precession photographs of timed duration were taken of the (hkO) and (h01) zones. The crystal then was mounted on a Weissenberg camera (Cu ILy radiation), and (Okl) was photographed by use of a multiple film pack. The intensities from the precession camera were measured visually. The optical densities of the Weissenberg reflections were read using a Welch Densichron, Model 10. The size and cylindrical nature of the crystal permitted absorption corrections to be ignored ( p varied from 0.16 to 0.17). Prior to the usual Lorentz and polarization corrections, two adjust(1) This paper was presented a t t h e 20th Annual Southwest Regional Meeting of t h e American Chemical Society, Dec. 3-5, 1964, Shreveport, La. (2) R. A. Zingaro, C. A. VanderWerf, and J. Kleinberg, J . A m . Chem. Soc., 7 3 , 88 (1951).
(3) H. Carlsohn, Angew. Chem., 46, 747 (1933).
Volume 69, Number 6 June 1966
1916
AUBREYJ. SEREWICZ, B. KEN ROBERTSON, AND EDWARD A. MEYERS
ments were made to the Weissenberg data. First, observed values of optical density obtained from the Densichron were brought into conformity with a standard scale supplied by the manufacturer. Second, this revised set of optical densities was used to obtain intensities by means of a very slightly modified form of the internal ciilibration method of S. H. Simonsen and P. A. Hom4 In this calibration method, the optical densities for the reflections of film n (highest intensity) are plotted as the abscissa z‘s. the corresponding reflections on film n + 1 as the ordinate. Then, on the same graph, film n + 1 is plotted as the abscissa us. the reflections of film n 2 as the ordinate. Through these points a best fit curve is drawn. .in arbitrary point on the curve in the high optical density portion on the abscissa is assigned an index of 100. The film scaling factor, c, is 1 the inused to give the corresponding point on n tensity of 100;~. This second optical density is then referred to the abscissa and the value read on the ordinate assigned an intensity of 100/c2. The process is continued to extinction. Through the use of the four to five points determined in this manner, a graph is constructed of optical density us. relative intensity. The fairly well-defined lower portion of the curve can be further used to complete the high intensity section. For the precession data, the scaling factor of 3.00 was used since this was the ratio between the various timed exposures. For the Weissenberg data, a value of 2.96 was used. This value was obtained from absorption measurements with a G.E. Spectrometer and is in good agreement with the value of 2.92 given in recent measurementsj reported on Ilford Industrial Type G film. The final intensities obtained in this way were corrected for Lorentz and polarization factors. It later became obvious that more extensive data were needed. Another single crystal (crystal 11)of the material was selected and mounted as previously described on the Weissenberg camera ; photographs (Cu Kcr radiation) were taken of the zero, first, and second levels with an integrating mechanism designed by Nordman.6 These data were treated as the earlier data had been, except that the film scaling factors were corrected for the inclination of the film in the first and second levels.
+
+
Analysis of the Structure The short a-axis suggested a single layer of molecules should be well resolved in the [loo] projection. A two-dimensional Patterson function was constructed with the (OM) data from crystal I. Examination of the Patterson map suggested a limited set of trial structures, which were systematically investigated. Only The Journal of Physical Chemistry
one of these refined satisfactorily, and trial y- and x-coordinates were obtained. The x-coordinates presented a problem because of the limited data collected from crystal I and because in the [OOl] and [OlO] projections the atoms were not well resolved. Packing considerations and difference Fourier maps were used to obtain preliminary values of the x-coordinates. With different scale factors and different over-all temperature factors for each zone, R factors of 0.21, 0.19, and 0.17 were found for the (hOZ), (hkO), and (OM) data, respectively. The least-squares program, OR FLS,’ was used to refine the data available from crystal I and confirmed the Fourier results, namely, that the z-coordinates had large uncertainties and the bond distances were consequently unreliable. Individual atom temperature factors did not improve R greatly. For these reasons, zero and upper level integrated Weissenberg data were collected for crystal 11. With individual isotropic temperature factors, the uncertainties in molecular parameters improved because of the more extensive data, but the bond distances remained unsatisfactory, and the R factor remained high (-0.18). A few structure factors for large reflections were removed from the least-squares calculations because it was believed that they were less accurate than the majority of reflections and that they were unduly biasing the results. The improvement was slight. The data from crystal I1 were used to calculate threedimensional Fourier and difference Fourier functions with the ERFR2 progranx8 In these maps, the hydrogen positions were indistinct, but there was clear indication of anisotropic thermal motion for the heavier atoms. An anisotropic least-squares refinement then was carried out with the Busing-Levy-Martin program,’ in which the best isotropic atom temperature factors obtained with unit weights for all reflections of crystal 11 were entered into OR FLS along with hydrogen positions calculated for C-H and ?\T-H bond lengths of 1.08 A. Immediate improvement established the importance of anisotropic thermal motion. RI = IFo F,\/ZIFo/ dropped to 0.088 and R2 = { ~ ~ c -I FFG 2 / ~/ ~ (4)
S.H .
Simonsen, University of Texas, private communication.
( 5 ) H. Morimoto and R. Uyeda, Acta Cryst., 16, 1107 (1963). (6) C. E. Nordman and -4. L. Patterson, Rea. Sci. Instr., 28, 384 (195i). (7) W.R. Busing, K. 0. Martin, and H. A. Levy, “ O R FLS, A Fortran Crystallographic Least-squares Program,” ORNL-TM-305, Oak Ridge National Laboratory, Oak Ridge, Tenn , 1962.
(8) R. G. Sly, D. P . Shoemaker, and J. H. Van den Hende, “Twoand Three-Dimensional Crystallographic Fourier Sumination Program for the IBM 7090 Computer,” CBRL-2251-62, Massnchusetts Institute of Technology-Esso Research and Engineering CO., 1962.
1917
CRYSTAL STRUCTURE OF PYRIDINE HYDROGEN NITRATE
ZZOF,~)~/~ to 0.098. The uncertainties in bond distances become approximately u = 0.012 8.,but all ring distances were shorter than anticipated. Weighting Schemes. The effect of two quite different weighting schemes was investigated. The f i s t of these was that developed by hug he^.^ In Hughes’ method all reflections less than 4F,i, are assigned a weight of [4Fmin/Fo]-2. Reflections above 4Fmin become weighted by [Fo/4F,i,]-2. A program incorporating this method was written for the IBM 709, and the weighted data were used for several leastsquares cycles. No improvement in R was observed. The bond distance relationship remained undisturbed. Table I : Atomic Coordinates from
Least-Squares Refinement
0(1) -0.4946 O(2) -0.6149 O(3) -0.3671 N(1) -0.1080 N(2) -0.4896 C(1) -0.0287 C(2) 0.1346 C(3) 0.2092 0.1248 C(4) C(5) -0.0305
0.0022 0.0025 0.0024 0.0024 0.0025 0.0034 0.0030 0.0029 0.0030 0.0032
0.4847 0.5572 0.4026 0.3131 0.4825 0.2230 0.1393 0.1462 0.2352 0.3199
0.0006 0.1157 0.0008 0.2545 0.2497 0.0009 0.0009 0.0465 0.0009 0.2093 0.0014 0.0990 0.0010 0.0554 0.0010 -0.0418 0,0013 -0.0934 0.0010 -0.0475
0.0007 0.0007 0.0007 0.0009 0.0008 0.0008 0.0010 0.0011 0.0008 0.0012
Thermal Motion and Bond Dzktances. Since the shortening of bond distances is one of the characteristics of certain types of anisotropic thermal motion,12-14 calculations were made to correct for discrepancy between the observed bond distances and the bond distances normally expected. Busing and Levy15 have pointed out that rigorous corrections would require a detailed analysis of the dynamics of the atomic system. It became necessary therefore to ascribe a reasonable simplified model to the system and to make approximate corrections. CruickshanklShas shown that the magnitudes of the atomic motions can be used to determine the rigid body vibrations of the molecule. It is assumed that the motion of a molecule can be expressed in terms of two symmetric tensors, one giving the translational vibrations of the mass center and the other giving the angular oscillations about the mass center. The model taken was that of treating the pyridine ring and the nitrate group as independent rigid bodies. If all the atoms in each rigid body are translated in the same direction, it is obvious that translation will have no effect upon the relative bond distance between atoms. Angular oscillations, however, will affect the positions of maxima in the density distribution. The corrections for the bonds in the nitrate group could be determined directly. With the assumption
Table 11: Least-Squares Anisotropic Temperature Factors ( X lo4)
1399 f 103 1214 f 108 1312 f 112 266 f 108 580 f 114 543 f 140 290 f 129 408 f 132 238 f 129 239 f 130
71f 8 126 f 10 138 i 11 75 f 11 57 f 10 120 f 16 61 f 11 62 f 12 112 f 14 48 f 11
38f 6 70f 7 62f 7 87 f 10 56f 9 27f 9 80 f 12 74 f 11 36& 9 98 f 12
The second, the self-consistent weighting scheme suggested by Cruickshank,lO was further developed in this 1aboratory.ll Used in conjunction with OR FLS, this method assigns to each reflection a weight of ( A Be F, CFo2)-L,where A , B, and C are fitted by least squares. Three such cycles gave RI = 0.085 and R2 = 0.094. The coordinates and anisotropic temperature factors from this refinement are given in Tables I and 11. The comparison of observed and calculated structure factors is listed in Table 111.
+
+
52 f 19 97 f 26 176 f 28 -49 f 21 -58 f 23 -107 f 32 -19 f 27 69 f 26 -51 f 31 -49 i 26
-6 -19 52 33 6 -34 -75 -32 -3 -38
i 16 f 19 f 19 f 21 f 21 f 22 f 25 f 24 i 20 f 26
o i -48f 45f -25f -5f
5
7
7 8 8 O f 9 22f 9 -27f 9 -5f 9 33 i 10
(9) E.W.Hughes, J. Am. Chem. SOC.,63, 1737 (1941). (10) D. W. J. Cruickshank, “Computing Methods and the Phase Problem in X-ray Crystal Analysis,” R. Pepinski, J. M. Robertson, and J. C. Speakman, Ed., Pergamon Press, New York, N. Y., 1961, p. 42. (11) R. F. Copeland and E. A. Meyers, paper presented at the 20th Annual Southwest Regional Meeting of the American Chemical Society, Dec. 3-5, 1964, Shreveport, La. (12) D. W. J. Cruickshank, Acta Cryst., 9, 747 (1956). (13) D.W.J. Cruickshank, ibid., 9, 754 (1956). (14) D.W.J. Cruickshank, ibid., 9, 757 (1956). (15) W.R. Busing and H. A. Levy, ibid., 17, 142 (1964).
Volume 69, Number 6 June 1966
AUBREYJ. SEREWICZ, B. KENROBERTSON, AND EDWARD A. MEYERS
1918
Table I11 : Observed and Calculated Structure Factors 0 0
k 0 0 0 0 1 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 8 8 8 8 8 8 9 9 9 9 10 11 11 12
1 2 4 8 10 2 3 9 0 1 3 4 5 8 9 12 1 2 3 5 8 9 13 0 1 2 S .j
d 7 8 !I 10 1
2 3 4 5 10 11 G 2 3 4 5 6 7 8 9 11 12 2 3 4 5 8 9 10 11 0 3 4 5 10 11 2 3 4 8 1 1 2 1
F, Fa 13.7 11.1 23.7 26.1 11.9 11.0 9.2 -5.7 10.3 -6.8 26.2 29.1 17.1 -13.9 3.7 3.0 12.1 10.7 11.9 -11.8 25.0 23.4 7.1 7.1 23.0 24.9 6.4 6.6 8.4 -6.1 23.8 -26.0 12.2 -10.4 27.1 28.8 17.3 16.7 8.2 6.1 14.7 -12.1 6.0 6.9 18.2 18.7 8.1 7.1 18.9 -18.2 7.8 5.9 11.7 -10.7 5.8 -5.6 7.1 -5.5 6.3 5.7 5.6 -5.7 23.0 24.7 30.7 -31.8 9.7 -9.3 7.1 -5.0 6.1 4.5 8.6 -5.4 6.9 -7.4 10.0 9.2 20.5 20.2 27.9 28.0 16.9 18.3 6.0 -3.6 8.8 -8.2 14.0 11.9 8.9 7.5 13.0 -13.2 8.2 -7.5 5.4 5.4 14.7 -15.9 14.0 -13.7 23.1 22.3 3.8 4.5 7.9 5.9 8.8 8.7 16.9 -18.5 5.4 -4.7 5.3 -5.9 7.0 5.1 7.9 7.7 12.3 -11.5 9.0 -8.0 11.3 9.7 8.1 9.1 6.7 -7.9 9.7 8.8 6.1 5.6 7.9 8.1 8.4 8.0 16.9 -17.9 9.6 -8.9 7.2 7.7
1 1
The Journal of Physical Chemistry
k 0 0 0 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 6 5 5 5 5 5 6 6 6 6 6 6 6 7 7 7 7 7 8 8 8 9 9 9 9 10 11
1 4 6 8 1 2 3 4 5 7 8 9 2 3 4 5 6 7 9 10 0 1 2 3 4 5 6 7 8 9 11 12 0 1 2 3 4 5 6 7 11 12 0 1 2 3 4 5 6 7 9 1 2 3 4 5 10 11 0 1 2 3 5 1 3 12 0 1 4 10 1 1
Fo 12.4 8.0 28.5 18.2 26.8 27.7 10.8 8.2 4.3 8.5 14.1 15.6 28.7 4.3 6.5 28.2 12.9 10.1 6.7 16.2 31.7 24.1 11.6 8.4 11.0 18.0 4.5 9.4 6.5 9.0 9.7 17.7 21.2 7.9 8.9 2.7 6.1 13.6 8.8 7.9 12.3 17.4 8.6 7.7 31.3 14.2 8.6 5.8 3.6 16.0 5.5 7.7 6.4 23.0 16.2 12.9 13.0 7.9 10.1 14.5 5.8 7.9 11.8 10.4 7.5 13.2 9.6 9.3 10.8 9.4 11.9
Fa -11.3 9.9 -29.9 18.6 -28.0 -27.1 8.7 -6.7 -3.6 8.3 13.6 -14.7 -29.3 -5.2 -5.7 29.0 12.6 9.8 -6.2 15.7 31.3 -24.3 -12.8 8.3 11.9 -16.6 -2.7 8.0 6.2 -8.9 10.2 -17.8 19.5 8.5 7.6 3.2 6.3 -12.0 -9.0 -8.5 12.8 16.7 9.8 6.1 -32.9 13.4 -6.9 -5.1 -5.8 16.5 3.9 6.6 -5.9 23.2 16.9 -12.5 -13.2 8.0 8.4 14.2 -4.2 5.2 -11.0 -10.7 7.4 -13.3 9.5 -9.5 11.1 -8.9 -12.6
1
1
0
0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 7 7 7 7 8 8 8 8 9 9 9 9 10 10 11
4 8 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 0 1 4 5 6 8 10 0 1 2 3 4 5 6 7 8 9 12 0 1 2 3 4 5 6 7 8 9 10 11 1 2 3 4 5 6 8 10 0 1 4 7 0 2 4 5 0 1 2 3 0 1 3
Fo
Fa
33.4 26.6 21.0 18.3 25.4 21.5 5.7 5.0 5.7 8.9 13.2 5.8 26.8 21.5 12.9 20.3 6.6 28.4 6.0 7.5 7.0 9.3 16.2 19.5 17.3 5.7 14.8 16.8 6.5 17.9 22.1 15.5 11.6 3.6 5.1 8.4 3.1 9.7 6.0 12.1 17.1 8.8 13.4 19.7 16.1 12.9 16.2 9.3 3.5 14.2 7.9 9.1 6.5 9.8 11.0 22.9 8.1 8.4 6.7 16.5 8.1 13.7 6.1 10.4 12.4 6.1 3.1 3.6 12.5 12.9 14.4 6.8 5.0 8.1 6.9
-34.4 -26.2 21.3 18.3 -26.7 -22.1 -4.3 2.4 -4.9 -8.8 13.4 6.0 24.9 21.3 11.1 19.4 -5.5 29.2 -3.7 -7.5 -6.1 -9.9 -15.7 18.6 -17.0 -5.0 14.6 -16.3 6.3 -17.8 -20.7 -16.2 -12.1 -3.1 3.6 -5.7 4.2 10.6 4.4 11.5 -16.7 -7.3 15.2 -18.3 -16.0 -12.9 -15.7 8.0 -5.8 15.3 8.9 9.4 -5.9 8.9 -11.5 22.4 6.8 -8.1 -7.2 -20.5 -8.0 12.7 -5.9 -10.4 -11.3 -4.6 3.8 2.8 13.3 15.4 14.9 -6.5 -5.1 -8.5 -7.1
2 2
k 0 0 0 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 5 5 5 5 5 6 6 6 7 7 7 7 8 8 8 8 9 9 9 9 10 10
1 2 6 10 1 4 5 6 7 9 11 12 13 0 1 2 3 4 5 6 7 8 9 10 11 12 0 2 3 4 5 6 7 8 9 10 0 1 2 3 4 6 8 9 0 1 3 5 6 0 1 8
0 1 2 4 2 4 6 7 0 1 2 8 0 1
Po 4.9 13.8 8.6 20.1 7.1 9.6 15.0 5.7 7.3 7.0 6.2 5.4 7.3 28.4 28.8 22.3 10.3 11.3 6.1 4.2 9.4 5.5 4.3 7.0 8.8 22.4 12.8 32.0 16.7 8.4 5.6 8.6 6.4 15.4 7.6 25.3 25.4 14.4 23.6 6.9 7.7 6.6 9.3 7.4 13.5 9.4 10.6 8.1 11.6 4.3 7.9 11.7 9.8 7.1 9.6 10.6 7.8 6.8 7.9 10.2 9.3 14.2 8.3 10.2 6.8
Fa 5.1 -13.4 8.7 -23.1 -5.1 -8.6 14.0 4.2 -5.4 7.2 -8.1 5.4 5.8 -29.8 -31.1 -23.1 -9.0 -8.4 4.2 2.9 9.4 5.4 2.8 5.3 -9.2 -22.1 12.7 33.2 -17.4 6.2 3.4 8.1 -6.6 -15.7 7.8 24.6 25.5 14.9 23.3 5.4 -6.0 -5.0 -8.7 -7.9 -12.5 8.7 -9.4 5.6 11.0 2.6 7.8 11.4 -9.5 4.9 8.1 -9.5 -5.7 -6.6 -7.2 -10.7 9.6 -15.7 7.7 9.3 6.4
2 2
F
l
F,
0 2 14.8 0 4 17.7 0 6 9.6 0 8 3.4 0 10 11.4 1 1 18.8 1 2 21.9 1 3 10.2 1 6 10.2 1 7 9.8 1 8 10.3 1 9 6.6 2 0 7.1 2 1 21.2 2 3 4.7 2 4 10.7 2 5 10.6 2 6 6.7 2 7 9.9 2 8 5.2 2 9 9.2 3 0 21.7 3 1 9.8 3 2 10.6 3 3 10.8 3 4 22.5 3 6 9.3 3 7 8.5 3 9 11.5 3 10 12.9 3 11 6.1 4 0 23.3 4 1 10.0 4 2 9.7 4 4 15.0 4 5 5.6 4 6 4.1 4 7 6.1 4 10 10.4 4 11 3.6 4 12 7.2 5 0 7.4 6 1 7.3 5 2 17.1 5 4 8.6 5 5 5.7 5 6 4.7 5 7 6.3 5 8 12.7 6 0 11.1 6 2 4.8 6 3 5.9 6 6 4.6 6 8 7.2 6 9 3.8 7 0 10.9 7 1 10.1 7 2 11.2 7 3 8.5 7 8 3.6 7 9 8.7 8 1 11.7 8 2 8.1 8 7 8.6 8 8 5.6 8 10 6.0 9 0 10.4 9 3 6.9 9 6 4.8 9 7 5.3 9 10 5.1 10 0 8 . 0
Fa -15.3 -18.8 -7.2 3.4 10.5 -19.7 -21.1 10.2 -9.2 8.8 9.2 -4.9 5.8 22.0 5.4 8.8 -10.4 5.3 -9.1 -3.8 -9.3 22.1 9.4 -10.8 10.2 23.7 -13.0 -8.0 -12.5 -13.1 -4.4 24.6 -8.8 -10.0 -15.3 4.7 -2.8 6.6 9.6 5.5 -6.4 7.9 -6.2 -16.1 7.4 4:2 -3.4 6.4 10.3 11.0 2.8 -4.9 -2.9 -6.3 4.2 -11.4 -9.2 -11.4 7.4 5.2 -8.6 12.7 -8.8 -8.8 6.6 6.0 10.7 5.4 -5.6 5.0 -4.6 9.3
CRYSTAL STRUCTURE OF PYRIDINE HYDROGEN NITRATE
that the three oxygens ride on the central nitrogen atom, the corrected bond distances could be found with the OR FFE program of Busing, Martin, and Levy.16 For the pyridine ring, the technique of Cruickshank13was employed to find the amplitude of the angular oscillations. Orthogonal unit vectors were set up in the molecule a t the center of the ring, with two of the vectors in the plane of the ring. Axis 1 was defined by the vector from N(1) to C(3). With the equations given by Cruickshank, the square amplitude of the angular oscillation about the axis normal to the planar pyridine ring could be approximated. All ring bond distances are affected by this oscillation, and it makes the principal contribution to the bond correction. Although the amplitudes about the remaining orthogonal axes could not be determined uniquely, upper and lower limits could be placed on their magnitudes. The cross terms, w , ~(i # j ) , were small enough to be ignored. The Levy and Busing correction for molecular libration’; was calculated. The uncorrected bond distances and upper and lower corrected values are given in Table
IV. Table IV : Bond Distances and Angles r
Uncor. dist., A.
--Cor. Upper limit
1.263 i 0.011 1.207 f 0.011 1.218 f 0.012 2.759 1 0 . 0 1 4 1.349 i 0.016 1.307 f 0.015 1.349 0.017 1.347 i 0.016 1.335 f 0.016 1.357 i 0.016
dist., bLower limit
1.292 i 0.013 1.258 i 0.013 1.273 f 0.013
...
1.379 1.337 1.384 1.377 1.363 1.390
+
Atoms
Angle, deg.
Atoms
N(l)-C(l)-C(Z) C(l)-C(Z)-C(3) C(2)-C(3)-C(4) C(3)-C(4)-C(5) C(4)-C(5)-N(l) c(5)-N(l)-C(l)
120.4 f 1.1 118.8 zt 1.1 120.3 + 1.1 120.0 I 1 . 1 120.1 zk 1 . 1 120.4 i 1 . 1
0(1)-N(2)-0(2) 0(1)-N(2)-0(3) 0(2)-N(2)-0(3) N(Z)-O(l)-N(l)
1.372 1.330 1.377 1.371 1.357 1.383
Angle, deg. 119.0 117.7 123.3 108.5
i 1.2 i 1.1 i 1.2 f 0.8
Discussion of Structure The Pyridine Group. The pyridine ring appears to be a nearly regular hexagon. Of the various ring bond distances that are expected to be the same, only the nitrogen-carbon bond distances seem possibly to be unusual. However, the difference between N(l)-C(l) and N(1)C(5) is only 0.042 d., and as the standard error in a ring bond distance is 0.016 8.,even this difference is not established as being significant. The final bond lengths have been taken as the averages of the values
1919
given in Table IV, following correction for anisotropic thermal motion. The mean values for bond angles and distances for the ring are given in Table V along with a number of values selected from various structures that have similar ring systems. The most accurate values in Table V are probably the microwave results for pyridine in the gas phase. Compared to benzene there is a shortening of two bonds in the ring, the N-C bonds, and a significant distortion of the bond angles. There has been an attempt to estimate bond angles for a number of different nitrogencontaining ring systems,17 but the results for pyridine are poor. The resultant bond angles are pictured as a compromise of the conflicting demands made by (1) repulsion of the electrons in the various atomic orbitals, (2) changing bond strength with altered hybridization, and (3) the energy required to promote the 2s electrons in nitrogen. For qualitative purposes, it seems easiest to interpret the distortions of the angle C(l)N(l)C(5) on the basis of the repulsion of the lone pair of electrons on the nitrogen atom18 causing a deformation of the ring of the kind that is observed. The experimental results for the ring systems in 2,2’-bipyridine are in good agreement with the microwave results for pyridine, and those of 2,2‘-pyridil are in fair agreement. When the nitrogen lone pair in pyridine is bonded to something else, it would be expected that the angle C(l)N(l)C(5) would increase. The various complex salts, Cuz(CH3CO2)4.2Py and SeOC12.2Py1seem to show this effect to some extent, along with an increase in the angle C(2)C(3)C(4) and a decrease in the angle C(1)C(2)C(3). In these structures, the bond distances Cu. * N(l) and Se- aN(1) are fairly long (between 2.12 and 2.19 d.). In the structure of pyridinoxide hydrogen chloride, a strong N(1)-0 bond of length 1.37 d. has been formed. The angle C(l)N(l)C(5) is 127O, while the angle N(1)C(l)C(2) has fallen to 116.6’. Similar behavior appears to be present in PyHCl and PyH[Cr(NCS)4(NH3)2], but in addition in these presumably ionic compounds, the angle C(l)C(2)C(3) has decreased markedly, and the angle C(2)C(3)C(4) has increased markedly although the variation among the various bond angles reported is large. On a simple ionic picture, the positive charge in the vicinity of nitrogen would be expected to produce this kind of distortion, increasing the angles C(l)N(l)C(5) and C(2)C(3)C(4) (16) W.R . Busing, K. 0. Martin, and H. A. Levy, “ O R F F E , A F o r t r a n Crystallographic Function and Error Program,” O R N L T M 306, Oak Ridge Laboratory, Oak Ridge, Tenn., 1964. (17) H . K i m a n d H . F. H a m e k a , J . Am. Chem. Soc., 85,1398 (1963). (18) R. J. Gillespie and R. S. Nyholm, Quart. Rev. (London), 11, 339 (1957).
Volume 69, Number 6 June 1966
AUBREYJ. SEREWICZ, B. KENROBERTSON, AND EDWARD A. MEYERS
1920
Table V 7
Bond,
%.
Angle, deg.
Compd.
PyHN03 Pyridine(g)" PyHClb PyH [Cr(NCS)4(NH,)zIe Cuz(CHsCO&.2Py orthorhombicd Cu9(CHsC02)a.2Pymonoclinic'
1.355 1.3402 1.32 1.35 1.330 1.378
1.383 1.3945 1.42 1.39 1.411 1.396
1.369 1.3944 1.40 1.40 1.359 1.422
120.4 116.8 128 134.0 119.2 117.2
120.2 123.9 118 114.5 122.0 123.4
119.4 118.5 115 112.7 117.5 115.0
120.3 118.3 125 131.7 121.6 120.0
' B. Bak, L. Hansen-Nygaard, and J. Rastrup-Andereen, J . Mol. Spectry., 2, 361 (1958). C. RBrat, Acta Cryst., 15, 427 (1962). Y. Takeuchi and R. Pepinsky, 2.Krist.,109,29 (1957). F. Hanic, D. StempelovA, and K. Hanicov6, Acta Cryst., 17,633 (1964). e G. A. Barclay and C. H. L. Kennard, J. Chem. Soc., 5244 (1961).
and so bringing the various electrons in the molecule closer to the site of the positive charge by distorting the ring angles. In PyHN03, all of the ring bond angles have been found close to 120". These angles were calculated for the uncorrected bond distances, but, because the ring is so nearly regular, only small changes would be expected if angle corrections were made. Moreover, Levy and Busing15 have pointed out that bond angle corrections should be made with caution. In PyHN03 both angles, C(l)N(l)C(5) and C(2)C(3)C(4), have increased compared to pyridine, but the changes are small compared to those reported for the two salt structures, PyHCl and PyH [Cr(NCS)d(NH&]. The Nitrate Group. The planar nitrate group shows a slight asymmetry, both in bond angles and distances, but again the errors are such that it is, at most, only suggestive. Of more interest are the corrected N-0 bond distances, which are fairly long compared to many earlier results. In Table VI the angles and distances for the nitrate group in PyHNOI are listed along with a number of values from structures that have similar units. From the recent microwave study of HNOa(g), it is seen that the N(2)-O(1) single bond, 1.405 A., is considerably longer than the multiple bond, N(2)-0(2) = N(2)-O(3) = 1.206 8. In addition, the angle O(2)N(2)0(3) is much larger than 120'. Values are also available for HNOa(s) but are complicated by the disorder present, in the crystal. The mean values selected differ appreciably from the gas phase results but are subject to large errors. In HNO3 HzO and HNO,. 3Hz0, some alterations in the various bond lengths and angles are found. N.m.r. work19 has strongly indicated that HN03. HzO is ionic and that it should be written as HsOfN03-. The estimated errors in the X-ray work are such as to prevent one from establishing with certainty that the bond disa
The Journal of Physical Chemistry
Table VI -Bond, Compd.
PyHNO3 HNOdg)" S)b
I.-
-Angle,
de&-
N(2)O(1)
N(2)-
N(2)O(3)
O(1)N- O(1)N- O(2)N(2)0(2)
1.292 1.405 1.30 1.29 1.26 1.243
1.258 1.206 1.24 1.20 1.22 1.243
1.273 1.206 1.24 1.24 1.23 1.243
119.0 117.7 123.3 114 116 130 113 113 134 114 119 127 119 119 122 By symmetry all 120'
HNOa.H20C HNOa.3H20d NzOse Methylguanidinium nitratef 1.243 1.235 1.254 120.3 119.9 119.5 Tris(ethylenediamine)nickel(11)nitrateg 1.21 Bis(ethy1enediamine)copper(I1) nitrateh 1.267 1.248 1.259 119.9 120.1 119.3 NaNOai 1.218 Pb( N03)z' 1.268 a D. J. Millen and J. R. Morton, J. Chem. SOC.,1523 (1960). V. Luzatti, Acta Cryst., 4, 120 (1951). V. Luzatti, ibid., 4, 239 (1951). V. Luzatti, ibid., 6, 157 (1953). E. Grison, K. Eriks, and J. L. de Vries, ibid., 3, 290 (1950). R. M. Curtis and R. A. Pasternak, ibid., 8, 675 (1955). L. N. Swink and M. Atoji, ibid., 13,639 (1960). Y. Komiyama and E. C. LingaR. L. Sass, R. Vidale, and J. felter, ibid., 17, 1145 (1964). Donohue, ibid., 10, 567 (1957). W. C. Hamilton, ibid., 10, 103 (1957).
'
' '
'
tances and angles are significantly different from those expected for a symmetrical nitrate group. The agreement between the dimensions of the nitrate group reported for NzOs and methylguanidinium nitrate is striking. In the first compoundb the crystal symmetry demands equal bonds of 1.243 A. and equal (19) R. E. Richards and J. A. S. Smith, Trans. Farada~/Soc., 47, 1261 (1951).
CRYSTAL STRUCTURE OF PYRIDINE HYDROGEN NITRATE
angles of 120'. In the second, there are no such symmetry requirements, but the dimensions are remarkably close to those found in NzOb. In both studies, the agreement bet,ween calculated and observed structure factors is good. Two similar nitrates containing divalent metal atoms are also given in Table VI. The values for bis(ethylenediamine) copper(I1) nitrate are probably more accurate than those for tris(ethylenediamine)nickel(II) nitrate. The mean N-0 bond length in the Cu(I1) complex is 1.258 8. Finally, two simple nitrates are listed. The difference in bond lengths is fairly large, but the estimated error in N-O in Pb(N0JZis 0.02 A. The average N-0 bond length in NaN03 is reported by the authors as being sensitive to the weighting scheme used and to the omission of reflections from the least-squares refinement. The corrections to bond lengths from torsional movements of the nitrate group in NaN03, using the r.m.s. amplitude given in the original paper, is very small (less than 0.01 A.). The mean bond length in PyHN03, following correction for anisotropic thermal motions, rose from 1.230 to 1.274 8.,and the variation in bond distances was reduced. Most of the recent structurtl evidence appears to favor a value of a t least 1.24 A. for the average N-0 distance in the nitrate group. The differences in bond distances in the nitrate group in PyHNO, do not appear to be significant, and only the angle 0(1)N(2)O(3) seems to be slightly larger than 120'. The nitrate group in PyHNOPthus appears to be a NOs- ion rather than an KNOBmolecule. Bonding between Pyridine and the Nitrate Group. The outstanding feature of the bonding between the pyridine ring and the nitrate group is the one relatively short bond of 2.76 f 0.01 8. between N(l) and O(1). All other contacts are 3.10 8. or greater. It is unfortunate that the hydrogen atom positions were not locatable from difference Fourier maps since the distance of 2.76 8. and the value of 108.5 f 0.8' for the angle N(2)0(l)N(l) are consistent with either an ionic bond of the type PyH+. .Nos- or with a hydrogen bond, N ( l ) .H-0(1). The near coplanarity of the pyridine ring and 0(1) is also consistent with either model. Because the hydrogen atoms were not found,
1921
the secondary differences in bond angles and distances for the ring and the nitrate group must be critically examined in order to argue that one or the other is present. The structural evidence cited earlier makes it seem most likely that P y H f - .NOa- is the correct model, and this could be confirmed by other experimental methods as was done for HN03 HzO. Another interesting feature of the crystal is its easy deformation by twisting around the a-axis. The configuration of the ions shown in Figure 1, with the pyridine ring and nitrate group in pairs rather than in some complex arrangement with cross linking between many different groups, makes twisting around the aaxis seem fairly reasonable.
-
0
c?Y Figure 1. Schematic projection of asymmetric unit on (100).
Acknowledgments. The financial support of the Robert A. Welch Foundation and funds from the Research Corp. used to purchase some of the equipment used in this work are gratefully acknowledged. The facilities of the Data Processing Center of the Texas A & M University System have been used extensively in the course of this research.
volume 69, Number 6 June 1966
