P. A. Mullen and M. K. Orloff
910 TABLE IV: DeviationsQof Atoms from Planes in A ill
Ot1) (O(2) O(3)
0.13 0.0
Na(l)
0.33
100
111
0.0
Na(2)
0.0
C(1) C(2) C(3)
100
0.0
3.05 2.76 1.9s
* A negative devia*ion indicates that the alom lies on the same side of the plane as the origin proach distance observed here is indicative of a moderately energetic interaction. The zeolite framework has undergone small framework distortions upon acietylene sorption, with respect to the dehydrated stateT2*which are qualitatively the same as those found for the 32 NH3 complexz3 of zeolite 4A, The largest angular changes have been of only -4" a t O(2) and 0 ( 3 ) , and the average Si or A l to 0 bond has increased by 0.01 even though no hydrogen bonding occurs. This latter increase has been considered to be an effect30 of hydrogen bonding in other structures. The Na(1) position has moved further rnto the large cavity by 0.13 A upon partial coordination by acetylene while the Na( 1)--0(3) ecreased slightly by 0.04 A. Actually the Na(l) position must be the average of two positions corresponding to associated and unassociated Na+ ions. The
Na(2) to oxygen approach distances have not changed significantly. The Na(3) to O(1) and to O(3) distances have increased dramatically to 3.4 A, 0.9 A more than the distances found in the dehydrated structure,z1 indicating strongly that this ion participates in the complexation of an acetylene molecule. Increases in these Na(3)-0 distances of 0.5 8, have been observed in the eight NH3 and the two or three trimethylamine complexes31 of zeolite 4A. In a study of the adsorption af acetylene and methylated acetylenes on dried y - a l ~ m i n aa, ~sorption ~ mechanism similar to that found in this work was suggested which involved A13+ ions and dimethylacetylene. In general, however, chemisorption with the elimination of a hydrogen atom was the principal sorption process when acetylene or methylacetylene were used.
Acknowledgment. This work was supported by the U. S. Army Research Office-Durham. We are also indebted to the NSF for their assistance (Grant No. GP-18213) in the purchase of the diffractometer, and to the University of Hawaii Computing Center. (30) G. Donnay and R. Allmann, Amer. Mineral., 55, 1003 (1970). (31) R. Y. Yanagida, M.S. Thesis, Universityof Hawaii, 1973 (32) M. M. Bhasin, C. Curran, and G. S. John, J. Phys. Chem., 74, 3973 (1970).
so rpt ion Spect rum of Pent a e ry t hrit oI Tetra nitrate P. A. Mullen* and M. K. Orloff Chemii:al Research Drvision, Amerrcan Cyanamrd Company, Stamford, Connecticut 06904 (Received November 27, 1972) Publication costs assisted by The American Cyanamrd Company
Thtk absorption spectrum of pentaerythritol tetranitrate (PETN) has been measured in acetonitrile from 39010 i o 1825 A. Absorption bands were observed at ea. 1935, 2600, and 2900 A. Molecular orbital calculations were performed to facilitate assignment of these absorption bands. The following assignments were made, 1935 ( X T* localized on the -NO2 groups), 2600, and 2900 A (n X* transitions of the -NO2 groups).
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The physical and chemical properties of the secondary explosive pentaerythritol tetranitrate (PETN), C(CH20N02)4, have been studied previous1y.l However, only limited spectral data i%rc?available based on work using a single crystal of PEE" which did not transmit below 2800 A.z In this paper an experimental and theoretical investigation of the electronic absorption spectrum of PETN is reported. Similar work on the secondary explosive hexahydro-1,3,5ttrinitro-s-tri azine has been published.3 A sample of PETN, available in this Laboratory as a slurry with 10% ethanol, was dried in uucuo. The absorption spectrum in Eastman spectrograde acetonitrile was obtained from 3900 to 2000 A using a Cary 14R spectrophotometer and from 2250 to 1825 A using a Jarrell-Ash vacuum scanning spectrometer. The vacuum ultraviolet The Journal of Physical Chemistry, Vol. 77, No. 7, 7973
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instrumentation and solution cell have been described previously.2, 3 The concentrations of PETN solutions were between and M . The melting characteristics of the PETN sample were obtained using a microscope hot stage (Mettler FP-2). The sample was heated at lO"/min up to 135.0", l.O"/min from 135.0 to 139.5". and at 0.2'1 min from 139.5' until nielt was completed. We observed three distinct particle shapes and a melting range from 139.5 to 140.4". PETN has been reported t o melt at 140141" with the pure compound melting at 141.3OO4 (1) T Urbanski, Ed , "Chemistry and Technology of Explosives,' Vol I i , Pergamon Press, New York, N Y , 1965, pp 175-185 (2) Reference 1, p 177 (3) M K Orloff, P A Mullen, and F C Rauch, J Phys Chem, 74, 2189 (1970) (4) P A Mullenand M K Orloff,J Mol Spectrosc 30, 140 (1969)
Absorption Spectrum of Pentaerythritol Tetranitrate
91 1
WAVELENGTH (AI
2900
2600
1936
-T-
The MO calculation predicts two strong electronic transitions at 48.00 (2080 A, f = 0.67) and 53.50 kK (1870 A, f = 0.16). On the basis of the calculation the 48.00-kK transition is associated with electrons localized on the -NO2 group in C~H50N02.The transition at 53.50 kK is calculated to be due to a A r* transition of the -NO2 group with a large contribution from an intramolecular charge transfer excitation involving the promotion of electrons from the C2HsO atoms to the -NO2 group. The calculation also predicts the existence of essentially forbidden (f =5 x delocalized transitions a t 23.00 (4350 A) and 28.60 kK (3500A). On the basis of the MO calculation we may consider that the broad prominent band in the spectrum of PETN at 1935 A represents two electronic transitions. These transitions are calculated to be a A r* transition localized on the -NO2 group (at 2080 A) and a transition of mixed character at 1870 A (64% A r* localized on the -NO2 group plus 36% intramolecular charge transfer). The calculated oscillator strength for the 2080-A transition is significantly larger than that of the 1870-A transition. Therefore, the observed band at 1935 A may be seen to arise primarily from A A* transitions localized on the -NO2 groups. The position, intensity, and character of this band are in agreement with experimental and theoretical results for ethyl nitrate.8 The weak bands observed in the spectrum of PETN at ca. 2900 and ca. 2600 A are assigned as the n A* transitions of the -NO2 group^.^
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30.0
45.0
40.0
45.0
60.0
55.0
WAVENUMBER (kK)
Figure 1. Absorption spectrum of PESN in
acetonitrile. The experimental solution spectrum of PETN is presented in Figure L ais molar extinction coefficient ( e ) in units of liters mole-1 cm-1 us. wave number (kK). We observed broad bands at ca. 34.48 (2900 A, 21.8), ca. 38.46 (2600 A, c; 75.3), and ca. 51.68 kK (1935 A, e 20,400). Molecular orbital (MO) calculations were carried out to facilitate assignments of these bands. MOI caEculations were performed on an individual ethyl nitrate moiety. This approximate approach of performing an MO calculation on part of a molecule in which the chromophoric groups are connected by a saturated linkage has been shown to be valid; e.g., in the study of polynitramimes.B The MO method employed was the CNDO/2 method of Del Bene and Jaffe6 as developed for excited states. The input geometry used in the MO calculation to specify the location od the atoms in the C2HsON02 molecule was taken from the crystal structure measurements of PETNa7All the valence electrons in the molecule and the 25 lowest singly excited states were included in the configuration interaction calculation of the absorption spectrum.
-
-
-
-
Acknowledgment. The authors acknowledge the encouragement and technical aid kindly provided by Dr. S. K. Deb.
(5) J . Stals, Trans. Faraday Soc., 67, 1739 (1977). (6) J. Del Bene and H. H. Jaffe, J. Chem. Phys., 46, 4050 (1968). (7) A. D. Booth and F. J. Lleweliyn, J. Chem. SOC.,837 (1947). (8) K. Kaya, K. Kuwata, and S. Nagakura, Bull. Chem. SOC, Jap., 37, 1055 (1964). (9) H. H. Jaffe and M. Orchin, "Theory and Applications of Ultraviolet Spectroscopy," Wiley, New York, N. Y., 1962, pp 182-184.
The Journal of Physical Chemistry, Vol. 77, No. 7, 7973
