2344
J . Phys. Chem. 1990, 94, 2344-2347
only with a molecular symmetry less than D2d These results confirm previous suggestions that the half-chromophore fragments are essentially planar in the case of 1 but nonplanar in the case of 6.1°
Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, the BASF Aktiengesellschaft in Ludwigshafen, and the Danish Natural Science Research Council for financial support.
Structures, Relative Stabilities, Barriers to Internal Rotation, and Vibrational Frequencies of Isomeric HNOH and HPOH Suk Ping So Chemistry Department, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received: August 2, 1989; In Final Form: October 10, 1989)
The geometries of isomeric HNOH and HPOH have been obtained at the SCF level by using various basis sets and their state energies have been corrected for electron correlation computed by MP3/6-3 1G**//6-31G** method. Vibrational frequencies for various isotopic isomeric HNOH and HPOH have been calculated. The trans conformers are predicted to be more stable than the cis conformers by 27.9 kJ mol-' for HNOH and by 6.1 kJ mol-' for HPOH. In contrast, reported MIND0/3 results indicate that cis-HNOH lies 23.0 kJ mol-] below rrans-HNOH. The cis and the trans barriers to internal rotation have been calculated to be 29.7 and 57.6 kJ mol-l for HNOH and 12.2 and 18.3 kJ mol-' for HPOH, respectively, and are all attractive dominant. In agreement with general experimental observations, the NO bond but not the PO bond has been found to have some double bond character.
Introduction Experimental studies of simple oxides and oxyhydrides of phosphorus are few. The combustion reactions of phosphorus and phosphorus-containing compounds are often accompanied by the emission of visible light. Species identified or suggested as giving rise to these emissions include PO, HPO, POz, HOPO, and (PO)z.1-5 The novel H 3 P 0 and H2POH isomeric species as well as HPO and phosphoric acid (HO)zHPO have been shown by infrared spectroscopy6 to be produced in the phosphine-ozone complex photolysis. Besides, HOOPO, H P ( 0 2 ) 0 , and metaphosphoric acid, HOP02, were also formed as secondary photolysis products in the reaction.6 Recently, Withnall and Andrew$ conducted an infrared investigation of argon/phosphine samples codeposited at 12 K with argon/oxygen samples passed through a microwave discharge. Their work confirms the earlier observation of PO and HPO by Lazilliere and J a c ~ x .It~ also provides an infrared spectroscopic evidence for the formation of additional species such as PO2, PO3, HOPO, P2OS,H3P0, and HPOH. The last five species were in fact observed for the first time. Among the various above-mentioned species, the structures of only PO, POz, and HPO have been determined experimentally.'"-12 ( I ) Fraser, M . E.; Stedmen, D.H. J . Chem. Soc., Faraday Trans. I 1983, 79, 527. (2) Fraser, M. E.; Stedman, D.H.; Dun,T. M. J . Chem. SOC.,Faraday Trans. I 1984, 80, 285. (3) Henchman, M.; Vigianno. A. A.; Paulson, J. F.; Freedman, A.; Wormhoudt, J. J . Am. Chem. Sot. 1985, 107, 1453. (4) Harris, D. G.; Chou, M. S.;Cool, T. A. J . Chem. Phys. 198582, 3502. ( 5 ) Hamilton, P. A.; Murrells, T. P. J . Chem. SOC.,Faruday Trans. 2 1985, 81, 1531; J. Phys. Chem. 1986, 90, 182. (6) Withnall, R.; Hawkins, M.; Andrews, L. J . Phys. Chem. 1986, 90. 575. (7) Whitnall, R.; Andrews, L. J . Phys. Chem. 1987, 91, 784. (8) Withnall, R.; Andrews, L. J . Phys. Chem. 1988, 92, 4610. (9) Lazilliere, M.; Jacox, M. E. NBS Spec. Publ. 1978, No. 561; J . Mol. Spectrosc. 1980, 79, 132. (IO) Butler, J. E.; Kawaguchi, K.; Hirota, E. J . Mol. Spectrosc. 1983, 101,
161. ( I I) Kawaguchi, K.; Saito. S.; Hirota, E. J . Chem. Phys. 1985.82, 4893. (12) Hirota, E.. unpublished work cited in: Lohr, L. L. J . Phys. Chem. 1984. 88, 5569
0022-3654/90/2094-2344$02.50/0
On the contrary, the geometries, energies, and vibrational frequencies of PO, POz, HPO, HOP (triplet state), PO3, H 2 P 0 , HOPO, HPO,, and HOPOz have been computed theoretically by using 6-3 1G plus polarization basis sets.I3J4 About 10 years ago, Loew et aLis performed MO calculations using the M I N D 0 / 3 method on a series of small molecules (namely NO, NO+, H3, and isomeric cations and neutral molecules of [H, N, 01and [H, H, N, 01)to explore ion-molecule reactions involving NO and NO+ which could lead to the formation of interstellar molecules. They found that the geometry-optimized but yet-unknown planar cis-HNOH was more stable than the trans conformer by 23.0 kJ mol-', contradicting the general observation that cis conformers are higher in energy owing to the positive steric effects. There has been no experimental or theoretical study on the structures and energies of HPOH reported. Withnall and Andrews8 thus suggested that cis-HPOH was more likely stable than its trans conformer by analogy to HNOH. Experimentally, trans-HONO has been foundi6to be lower in energy than cis-HONO by 2.1 f 0.8 kJ mol-I. However, ab initio calculation^^^^^^ on this molecule have shown that the energy difference AE (=Ecis- E,,,,,) between the cis and the trans conformers depends strongly on the levels of sophistication of theory and the basis sets used, with values ranging from -18.6 to + ] O S kJ mol-]. It is interesting to note that only the medium-sized economical 4-31G basis set gives the best agreement with experiment ( A E = +2.69 kJ mol-I). Thus, a theoretical study on the geometries, relative stabilities, barriers to internal rotation, and infrared spectra of isomeric HNOH and HPOH was carried out in the hope of getting an insight to their electronic structures as well as supplying some (13) Yabushita, S.; Gordon, M. S . Chem. Phys. Lett. 1985, 117, 321. (14) Lohr, L. L.; Boehm, R. C. J . Phys. Chem. 1987, 91, 3202, and references cited therein. (15) Loew, G. H.; Berkowitz, D. S.; Chang, S.Astrophys. J . 1978, 219, 458. (16) Cox Peter, A.; Brittain, A. H.; Finnigan, D. J. J . Chem. SOC.,Faraday Trans. 1971, 67, 2179. (17) Turner, A. G. J . Phys. Chem. 1985, 89, 4480. (18) Darsey, J. A.; Thompson, D. L. J . Phys. Chem. 1987, 91, 3168.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 6. 1990 2345
Geometries of Isomeric H N O H and HPOH
TABLE I: Optimized Geometries and Energies of HXOH (X = N,P) for Various Basis Sets" LHOX
E,
AE
111.9 1 15.0 115.4 110.2 111.7 11 1.8 112.0
-129.64081 -130.173 06 -1 30.306 93 -129.723 38 -130.244 36 -130.368 76 -130.378 64 (-130.71804)
39.3 39.2 39.0 28.3 29.5 29.5 28.0 (27.9)
trans-HNOH 100.0 101.8 102.1 100.9 101.7 101.8 101.9
105.0 107.9 108.3 103.7 105.4 105.5 105.7
-129.655 77 -1 30.1 87 98 -130.321 80 -129.734 14 -130.255 59 -130.379 99 -130.38931 (-130.728 68)
0 0 0 0 0 0 0 (0)
0.965 0.948 0.950 0.964 0.947 0.947 0.943
cis-HPOH 99.3 97.8 97.8 100.0 98.3 98.9 99.0
120.4 124.8 122.6 122.2 116.6 116.2 116.6
-414.574 16 -416.201 62 -416.64678 -414.681 66 -416.29201 -416.718 53 -416.72771 (-417.021 43)
8.1 5.4 9.2 2.6 3.0 5.3 4.9 (6.1)
0.966 0.949 0.95 1 0.965 0.948 0.948 0.944
trans-HPOH 94.9 92.7 93.3 95.6 94.2 95.0 95.2
115.2 119.1 117.8 116.4 111.9 111.7 1 12.0
-414.577 26 -416.20366 -416.650 30 -414.68264 -416.293 17 -416.72055 -416.729 57 (-417.023 75)
0 0 0 0 0 0 0 (0)
r(XO)
r(XW
r(OH)
3-21G 4-31G 6-31G 3-21G* 4-31G* 6-31G* 6-31G**
1.427 1.391 1.385 1.357 1.346 1.346 1.345
1.030 1.019 1.019 1.024 1.013 1.013 1.014
0.970 0.955 0.954 0.965 0.950 0.950 0.946
LHXO cis-HNOH 106.1 107.7 108.0 106.3 106.9 106.9 107.0
3-21G 4-31G 6-3 1 G 3-21G* 4-31G* 6-31G* 6-31G**
1.433 1.396 1.389 1.360 1.348 1.348 1.347
1.024 1.012 1.012 1.021 1.010 1.010 1.010
0.967 0.952 0.951 0.964 0.949 0.949 0.945
3-21G 4-31G 6-31G 3-21G* 4-31G* 6-31G* 6-31G**
1.690 1.705 1.729 1.632 1.640 1.643 1.641
1.443 1.464 1.446 1.416 1.423 1.414 1.416
3-21'3 4-31G 6-31G 3-21G* 4-31G* 6-31G* 6-31G**
1.698 1.721 1.737 1.640 1.646 1.646 1.645
1.422 1.443 1.427 1.401 1.411 1.403 1.405
'All bond lengths are in angstroms, bond angles in degrees, total energies E, in hartrees, and relative energies AE in kJ mol-'. MP3 energies are in parentheses.
means for their future identification. Calculations The structures of H N O H and HPOH were optimized by the energy gradient method at the SCF level using GAUSSIAN 82 and GAUSSIAN 88 program^'^ implemented on our IBM4381 and Micro VAX2000 computers, respectively. The basis sets employed are the split-valence 3-21G, 4-31G, 6-31G, 3-21G*, 4-31G*, 6-31G*, and 6-31G**. In the study of the barriers to internal rotation of the OH group about the XO (X = N, P) bond, the geometries of H N O H and HPOH at different fixed internal rotation angles T were fully optimized by using the 6-3 1G** basis functions. The energies of the various species at the optimized 6-31G** geometries were then recalculated by the third-order perturbation theory with the Mailer-Plesset (MP3) partitioning of the Hamiltonian20 in order to take into account the electron correlation. Harmonic vibrational frequencies were computed by analytically differentiating the energies twice. Results and Discussion Geometry optimization yields a planar structure for the cis and the trans conformers of H N O H and HPOH. The molecular dimensions and energies are listed in Table I. As expected, the 0 9 ) . (a) Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; Whiteside, R. A,; Schlegel, H. B.; Fluder, E. M.; Pople, J. A. GAUSSIAN 82; Carnegie-Mellon University: Pittsburgh, PA, 1983. (b) Frisch, M. J.; Head-Gordon, M.; Schlegel, H. B.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A,; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Fluder, E. M.; Topiol, s.; Pople, J . A. GAUSSIAN 88; Gaussian, Inc.: Pittsburgh, PA, 1988. (20) Moller, C.; Plesset, P. S. Phys. Reu. 1934, 46,618.
bond angles of the cis conformers are seen to be. larger than those of the trans conformers (by about 4-7'). Data of Table I show that the N O bond of HNOH and the PO bond and the HOP angle of HPOH are most sensitive to basis sets. As the basis set is improving from 3-21G through 4-31G, 6-31G, 3-21G*, 4-31G*, and 6-31G* to 6-31G**, the N O bond shortens from about 1.43 to about 1.35 A, while the PO bond varies from about 1.69 to about 1.64 A through a maximum value of about 1.73 A (6-31G), and the H O P angle changes from 120.4°/115.20 (for cis/trans HPOH, respectively) to 116.6°/112.00 with a maximum of 124.8'/119.1' given by the 4-31G basis functions. However, it is important to note that the molecular dimensions remain practically the same for the 4-31G*, 6-31G*, and 6-31G** basis sets. Hence, it is unlikely that any further improvement on the basis set will change the geometries significantly. For all the basis sets employed, the optimized H X O angle of H N O H is larger than that of HPOH by about 6-10' while the HOX angle behaves just in the opposite way. This is in line with Gillespie's VSEPR theory.21 The two most stable conformers of H N O H and HPOH are found to be the cis ( T = 0') and the trans ( T = 180') forms. Unlike in the case of HON0,I6 our calculations with different basis sets all result in the cis conformer lying energetically above the trans one (Table I). This disagrees with the M I N D 0 / 3 resultls which predicted that cis-HNOH was more stable than trans-HNOH by 23.0 kJ mol-'. The energy difference AE between the cis and the trans conformers obtained in this work varies from 27.9 to 39.3 kJ mol-' (MP3/6-31G** value = 27.9 kJ mol-') for H N O H and from 2.6 to 8.1 kJ mol-' (MP3/6-31G1* value = 6.1 kJ mol-') for HPOH. (21) Gillespie, R. J. J . Chem. Educ. 1963, 40, 295.
2346
The Journal of Physical Chemistry, Vol. 94, No. 6,1990
TABLE 11: Calculated Vibrational Frequencies (em-') and Infrared Intensities (km mol-') of IsotroDic HNOH and HPOH" descrbtion' svm %.H l80.H l60.D l8O.D a" a' a' a' a' a'
481 (11) 1238 (83) 1643 (60) 1450 (8) 3597 (20) 4130 (43)
480 (10) 1205 (80) 1635 (56) 1449 (9) 3597 (20) 4116 (42)
362 (5) 1230 (98) 1272 (8) 1030 (2) 2632 (12) 3010 (26)
359 (5) 1194 (94) 1265 (6) 1030 (2) 2632 (12) 2990 (25)
a" a' a' a' a' a'
724 (254) 722 (253) 528 (135) 526 (1 34) 1240 (77) 1207 (82) 1223 (58) I195 (54) 1705 (4) 1699 (4) 1298 (22) 1287 (13) 1379 (144) 1376 (132) 1006 (76) 1001 (81) 3662 (3) 3662 (3) 2679 (2) 2679 (2) 4162 (88) 4148 (87) 3031 (49) 3012 (48)
a" a' a' a' a' a'
208 (110) 208 (108) 154 (61) 860 (187) 890 (177) 893 (207) 972 (61) 972 (52) 688 (43) 1178 (55) 1184 (57) 899 (31) 2489 (225) 2489 (225) 1789 (120) 4203 ( 1 15) 4189 ( 1 1 1 ) 3061 (76)
a" a' a' a' a' a'
403 (122) 402 (121) 294 (66) 292 (65) 848 (59) 892 (1 36) 860 ( 1 37) 870 (24) 988 (37) 712 (12) 713 (11) 990 (49) 1203 (153) 1197 (148) 918 (227) 899 ( I 75) 2568 (153) 2568 (1 53) 1846 (81) 1846 (81) 4185 (126) 4171 (124) 3047 (75) 3028 (72)
153 (60) 856 (162) 688 (43) 892 (31) 1789 (120) 3041 (72)
a Molecules containing ISN or mixed hydrogen isotopes are not considered. Intensities are in parentheses. Frequencies listed are unscaled values. T . P. and 6 denote the twisting, stretching, and bending modes, respectively.
'
TABLE Ill: Observeds Fundamental Frequencies (cm-') of Isotopic HPOHa description
r(HP0H) 4PO) 6(HPO) 6(POH)
sym a" a' a' a'
4OH)
a'
160,H 406 817,7 955.7 1094.4
180,H 405 788.1 955.3 1089.6
160,D 303 816.6 710.5 821.8
'*O,D
2689.0
2673.0
302 787.3 710.5 814.7
3'
OSee footnote of Table 11.
The vibrational frequencies and infrared intensities of isotopic HNOH and HPOH (molecules containing ISN or mixed hydrogen isotopes are not considered) have been computed by using the 6-31G** basis set and are listed in Table 11. The fundamental frequencies of isotopic HPOH observed by Withnall and Andrewss are reproduced in Table 111 for comparison purposes. The agreement between theoretical and experimental vibrational frequencies is generally improved by scaling the theoretical values. The scale factor corrects both for the deficiencies in the theory and for the neglect of anharmonicity. Recently, the vibrational frequencies of quite a number of molecules have been computed at various levels of theory and compared with experiment.22 The effect of scaling has been investigated. For 92% of the frequencies examined, the HF/6-3 lG* harmonic frequencies uniformly scaled by a factor of 0.89 lie within 100 cm-' of experiment with a mean absolute error of 49 cm-I. To our belief, such a conclusion would be still valid for the somewhat more extended 6-31G** basis set. When uniformly scaled by a factor of 0.89, the calculated vibrational frequencies of trans-HPOH in Table I1 compare favorably with the observed values in Table 111. The agreement is within the expected accuracy, and the vibrational frequencies of the isotopic HNOH conformers after being scaled in this manner should therefore be rather realistic. However, as can be seen from Tables 11 and 111, the agreement between the theoretical frequencies of cis-HPOH and the observed values is not quite as good. In particular, the unscaled theoretical twisting frequencies are only about half the observed values. Therefore, on this basis, (22) DeFrees. D.J.; McLean, A. D. J . Chem. Phys. 1985, 82, 333
so TABLE IV: Optimized Bond Lengths (A), Bond Angles (deg), and Energies (au) of HNOH at Different Fixed Internal Rotation Angles T (de&' T r ( N 0 ) r(NH) r ( 0 H ) LHNO LHON E, 0 (cis) 1.345 1.014 0.946 107.0 112.0 -130.37864 30 1.349 1.015 0.946 106.7 111.3 -130.37703 60 1.363 1.016 0.948 105.4 109.5 -130.37281 80 1.375 1.016 0.948 104.3 107.9 -130.37081 82.49 1.375 1.016 0.948 104.2 107.7 -130.37076 83.67 (TTS) 1.376 1.016 0.948 104.2 107.6 -130.37075' 90 1.377 1.016 0.948 103.9 107.1 -130.37094 100 1.377 1.015 0.947 103.6 106.5 -130.37202 120 1.369 1.013 0.946 103.1 105.9 -130.37669 150 1.353 1.011 0.945 102.4 105.7 -130.38543 180 (trans) 1.347 1.010 0.945 101.9 105.7 -130.38931
For the exact torsional transition state (TTS), T also has been allowed to vary in the geometry optimization. 'The MP3 value is -I 30.706 73 au. TABLE V: Optimized Bond Lengths (A), Bond Angles (deg), and Energies (au) of HPOH at Different Fixed Internal Rotation Angles T (deg)' T r ( P 0 ) r(PH) r ( 0 H ) LHPO LHOP E, 0 (cis) 1.641 1.416 0.943 116.6 -416.72771 99.0 30 1.640 1.417 0.944 99.2 116.5 -416.72727 60 1.642 1.418 0.945 99.6 116.0 -416.72577 90 1.646 1.416 0.945 99.3 115.2 -416.72445 91 81 1.646 1.416 0.945 99.4 115.1 -416.72443 93.83 (TTS) 1.646 1.416 0.945 99.2 115.0 -416.72443b 120 1.646 1.411 0.944 98.0 113.7 -416.72548 150 1.645 1.407 0.944 96.1 112.5 -416.728 16 180 (trans) 1.645 1.405 0.944 95.2 112.0 -416.72957 aSee footnote a of Table IV. 'The MP3 valve is -417.01679 au.
the observed bands are attributed, in contrast to the assignment of Withnall and Andrews,* to the trans-HPOH conformer even though the barrier separating these two conformers is not high (see below). This is consistent with our prediction that transHPOH is lower in energy than cis-HPOH by 6.1 kJ mo1-l (MP3/6-31G**). The PH stretch of the four isotopic HPOH and the OH stretch of the two nondeuterated HPOH molecules have not been observed.8 Our calculated infrared intensities for the bands associated with these vibrations show that this is most likely due to the masking by other bands rather their weak intensity. To study the barrier to internal rotation of HXOH (X = N, P), the geometries of the conformers at different fixed internal rotation angles T have been optimized by using the 6-3 lG**basis set and are listed in Tables IV and V. The most significant changes in molecular dimensions caused by the internal rotation of the OH group about the XO bond involve the NO bond of HNOH and the two bond angles of both HNOH and HPOH. As T increases from 0' to 180°, the HXO and the HOX angles both decrease monotonically, probably owing to the decreasing steric effect of the two terminal hydrogen atoms. The NO bond goes through, at a T value of about 90°, a maximum 0.032 and 0.030 A longer than those of the cis and the trans conformers, respectively. The other bonds of HNOH and HPOH also show a similar variation but by a negligibly small amount. Note that the NO bond length is much shorter than would be expected for a NO single bond (e.g., 1.453 A in NH,OH), indicating some partial double bond character. In addition, the relatively large variation in this bond length indicates the influence of delocalization in the planar conformers which is probably responsible for the energy barriers (29.7 and 57.6 kJ mol-' for the cis and trans barriers, respectively, see below) being higher than would be expected for rotation about a "typical" single bond (12.0-23.9 kJ The negligibly small variation of the PO bond length with T and the very low barrier to internal rotation (12.2 and 18.3 kJ mol-', see below) suggest that the PO bond of HPOH has (23) Cotton, F. A,; Wilkinson, G . Adoanced Inorganic Chemistry; Interscience: Sew York, 1972; p 368. (24) Durig, J. R.; Harlan, R . J.: Groner. P. J. Phys. Chem. 1989, 93, 3041.
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2341
Geometries of Isomeric H N O H and HPOH TABLE VI: Mulliken Charge Distributions and Total Atomic Overlap Populations of HNOH and HPOH HNOH HPOH HXOH (X = N, P) 0' 83.67' 180' 0' 93.83' 180' Charge Distribution H(-X) +0.256 +0.271 +0.287 -0.129 -0.119 -0.090 X -0.124 -0.110 -0.163 +0.485 +0.487 +0.440 0 -0.480 -0.517 -0.491 -0.716 -0.730 -0.711 H(-0) +0.348 +0.356 +0.367 +0.360 +0.361 +0.361
H-X X-0 0-H
Overlap Population +0.606 +0.606 +0.641 +0.499 +0.450 +0.530 +0.222 +0.177 +0.211 +0.295 +0.313 +0.311 +0.613 +0.610 +0.631 +0.630 +0.630 +0.639
nearly no double bond character. This is consistent with the rather long PO bond predicted: 1.641 8, in cis-HPOH and 1.645 8, in trans-HPOH for 6-31G** compared with 1.467 8,in PO2'' and 1.6 8, for the sum of the single-bond radii. The above difference between the N O bond of H N O H and the PO bond of HPOH is also evidenced by the variation of the total atomic overlap populations with T as listed in Table VI and is due to the fact that nitrogen can form strong pr-pr bonds while there are no such bonds known in phosphorus compounds.23 Indeed, this latter fact leads to the existence of P(OR), but not of N(OR),, nitrogen giving instead O=N(OR). A least-squares fit using a few SCF 6-31G** energies (three for HPOH and four for HNOH) around the barrier top gives the latter at T = 82.49' for H N O H and T = 91.81' for HPOH. (Nitrous acid25and the hydroxy form of monothioformic acid (HCSOH)26also have their barrier tops quite a few degrees away from the 90' conformer.) The exact torsional transition states (TTS) or barrier tops of H N O H and HPOH were then obtained by full geometry optimization (Le., T also allowed to vary) using the structures of these approximate barrier top conformers as initial guesses. The resulting molecular dimensions are found to be practically the same as the starting ones except T (even which increases by 1 '-2' only; see Tables IV and V). This indicates that the true reaction coordinate for the cis-trans isomerization of HNOH and HPOH is practically equal to the internal rotation angle. Consequently, the cis and the trans barriers to internal rotation are respectively 29.7 and 57.6 kJ mol-' (or 20.7 and 48.7 kJ mol-] without correlation correction) for HNOH, and 12.2 and 18.3 kJ mol-' (or 8.6 and 13.5 kJ mol-I without correlation correction) for HPOH. The much smaller barrier to internal rotation of HPOH indicates that the internal rotation of its O H group about the PO bond is very much less restricted (e.g., 11.5 kJ mol-' for CH,CH,). The prediction of an imaginary vibrational frequency for the torsional transition state of H N O H (738i, 1156 ~
(25) Skaarup, S.; Boggs, J. E. J . Mol. Struct. 1976, 30, 389 (26) So, S. P. J . Mol. Struct. 1987, 151, 141.
TABLE VII: Calculated 6-31G** Total Energy and Its Components (au) for HNOH and HPOH 0' (cis) 90' (TTS)' 180' (trans) HNOH -130.37864 -130.37075 -130.38931 (-1 30.72868) (-130.71804)b (-130.70673) 130.08798 130.08302 130. IO068 -377.43661 -377.22352 -376.27965 82.29722 82.1 71289 8 1.759607 34.649464 34.585613 34.066271 1.00110 1.00110 1.001 IO -416.72771 (-41 7.02143) 41 6.36502 -1 100.8659 213.36422 54.408933 1.00045
HPOH -416.72443 (-41 7.01 679) 416.36357 -1 100.5654 213.23389 54.243462 1.00045
-416.72957 (-41 7.02375) 416.36972 -1 100.9392 21 3.41947 54.420441 1.00045
"83.67' for HNOH and 93.83' for HPOH. bMP3 values are in parentheses. V = V,, + V, + Vnn.
1344,1555,3562, and 4109 cm-I; or 7323,1158,1344,1557,3561, and 4109 cm-l for the approximate TTS) and of HPOH (391i, 873, 1029, 1064,2491, and 4171 cm-'; or 3863, 872, 1030, 1061, 2489, and 4171 cm-' for the approximate TTS)confirms that they are not true equilibrium structures. The total energy ( E , ) and its various components, namely electronic kinetic energy (T),electron-electron repulsion energy ( Ve), nuclear repulsion energy ( Vm), and electron-nuclear attaction energy (V,,), for the cis, 90°, and trans conformers of H N O H and HPOH are listed in Table VII. Results in this table show that as the cis and the trans conformers of HXOH (X = N, P) change to the 90' one, the increase in the attractive energy (Vne) is larger than the decrease in the repulsive energy (V,,, = T + V, + Vnn). Hence, the cis and the trans barriers to internal rotation of these molecules, like those of mono- and dithioformic acids26g27and chloroformyl thiohypochlorite,28 are both attractive dominant.29 This conclusion is found to be unaffected by scaling to satisfy the virial theorem.,O It is interesting to note25*28 that the barrier to internal rotation is attractive dominant for cisH O N O and anti-CICOOCI but repulsive dominant for transH O N O and syn-ClCOOCl. Acknowledgment. I thank Thomas Tsui of the Chinese University Computer Services Centre for his assistance in implementing the GAUSSIAN 88 package on our Micro VAX2000 computer. (27) So, S . P. J . Mol. Struct. 1986, 148, 153. (28) So, S. P. J . Mol. Struct. 1988, 168, 217. (29) Allen, L. C. Chem. Phys. Lett. 1968, 2, 597. (30) Lowdin, P. 0. J . Mol. Spectrosc. 1959, 3, 46.
