5241 The reverse may be the case in B and C since here the product (boroxine or trihydroxylboroxine) is potentially aromatic. However, the available evidence suggests that the aromatic stabilization energy is much less than that in benzene (20 kca 1/mol). 2 5 In D-F, the inductive effects of the alkyl groups should stabilize the esters and so tend to make AH (and hence h ) more negative. This is supported by the fact that the heat of reaction of ethanol with boric acid (F) is more negative than that of methanol (E). In the conversion of boronic acid to 1, neither resonance nor inductive effects should significantly affect the heat of reaction. From the examples above, it would seem that the value of h in such circumstances should be greater than zero but less than 7 kcal/mol. Since eq 1 involves loss of water from three separate pairs of O H groups, the corresponding heat of reaction should then lie between 0 and 21 kcal/mol; using this value and the heats of formation cited above, we find: -165.3
< AHr(1) < -144.3
kcal/mol
(2) This agrees reasonably well with our MNDO estimate (-142.9 kcal/mol) but is very much less than the claimed experimental value (-200.4 kcal/mol; Table IV). It seems clear that the latter must be grossly in error and that the best available estimate of the heat of formation of 1 is -155 f 10 kcal/mol.
(3) P. K. Weiner, Ph.D. Dissettatlon, The University of Texas at Austin, 1975. (4) M. J. S.Dewar, Chem. Br., 11, 97 (1975). (5) See J. A. Pople and D. L. Beverldge. "Approximate Molecular Orbital Theory", McGraw-Hill, New York, N.Y., 1970. (6) (a) J. A. Pople. D. L. Beveridge. and P. A. Dobosh, J. Chem. phys., 47,2026 (1967); (b) J. A. Pople, D. P. Santry, and G. A. Segal, ibid., 43, S129 (1965). (7) M. J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 90, 4899 (1977). (8) M. F. Guest, J. B. Pedley, and M. Horn, J. Chem. Thermodyn.. 1, 345 (1969). (9) L. Barton, S. K. Wason, and R. F. Potter, J. Phys. Chem., 69, 3160 (1965). (10) G. L. McKown, B. P. Don, and R. A. Beuadet, Chem. Commun., 765 (1974). (1 1) E. A. McNeill, K. L. Gallaher. F. R. Scholer, and S.H. Eauer, lnorg. Chem., 12, 2108 (1973). (12) R. A. Beaudet and R. L. Poynter, J. Chem. Phys., 53, 1899 (1970) (13) J. P. Pasinski and R. A. Beaudet, J. Chem. Phys., 61, 683 (1974). (14) M. J. S. Dewar and H. S.Rzepa, to be published. (15) M. A. Weiner and M. Lattman, lnorg. Nucl. Chem. Lett., 11, 723 (1975). (16) E. A. Laws, R. M. Stevens, and W. N. Lipscomb. J. Am. Chem. SOC.,94, 4461 (1972). (17) W. C. Ermler, F. D. Glasser, and C. W. Kern, J. Am. Chem. Soc., 96,3799 (1976). (18) T. K. Ha, J. Mol. Struct., 30, 103 (1976). (19) M. W. P. Strandberg. C. S.Pearsall, and M. T. Weiss, J. Chem. Phys., 17, 429 (1949). (20) A. Veillard and R. Daudel, "La Nature et Les Proprietes des Liaisors de Coordination",Collogues lntemationauxdu Centre National de !a Recherche Scientifique, No. 191, Paris, 1970, p 23. (21) C. Cone, M. J. S. Dewar, and D. Landman, J. Am. Chem. Soc., 99, 372 (1977). (22) J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Draxl, and F. H. Field, Mat/. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 26 (1969). (23) Thus the BO bond length (1.38 A) in boroxine (H&0 ) is if anything qeater than in open chain boronic anhydrides (e.g., 1.36 in MenBOBMe?)(for references see Table V). (24) D. R. Stull and H. Prophet, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., NaN. Bur. Stand., No. 37 (197 1). (25) See M. J. S. Dewar and C. de Llano, J. Am. Chem. Soc., 91, 789 (1969). (26) R. C. Rastogi and N. K. Ray, lnt. J. Quant. Chem., 11, 435 (1977).
1
References and Notes (1) Part 39: M. J. S. Dewar and W. Thiel, J. Am. Chem. SOC., 99, 4907 (1977). (2) (a) R. C. Bingham, M. J. S.Dewar, and D. H. Lo, J. Am. Chem. Soc.,97, 1285 (1975):(b) ibid., 97, 1294, 1302, 1307 (1975); (c)M. J. S.Dewar, D. H. Lo, and C. A. Ramsden. ibid., 97, 1311 (1975).
CNDO Treatment for Faraday B Terms of Some Azaheterocycles Akira Kaito, Masahiro Hatano,* and Akio Tajiri Contributionfrom the Chemical Research Institute of Non-aqueous Solutions, Tohoku University, Sendai 980. Japan. Received December 27, 1976
Abstract: The magnetic circular dichroism (MCD) spectra of pyridine, pyridazine, pyrimidine, pyrazine, and 1,3,5-triazine
were measured in the wavenumber region of 25000-50000 cm-I. The transition energies, the oscillator strengths, and the Faraday B terms were calculated within the framework of the CNDO/S-CI approximation. The agreement between theoretical and experimental results is satisfactory. The perturbing mechanism for the Faraday B terms of the lowest a* n and the lowest s* a transitions were clarified on the basis of the calculated results.
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The value of the magnetic circular dichroism (MCD) technique as a useful tool of spectral and molecular structural analysis has been confirmed extensively by an introduction of the quantum mechanical treatments of the Faraday parameters. It has been shown that the Pariser-Parr-Pople (PPP) method' has explained well the experimental Faraday A terms of aromatic organic compound^,^-^ which originate from the Zeeman splitting of the ground or excited electronic state. The Faraday B terms arising from the magnetic mixing among electronic states have also been successfully interpreted6-8 by the PPP method. Although the PPP method can easily treat a* x transitions of large aromatic systems within relatively short computer time and small computer capacity, several authors have pointed out the importance of an inclusion of all valence electrons in the molecular orbital treatment of the
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Faraday p a r a m e t e r ~ . ~ The - l ~ CNDO method,l3.I4 a semiempirical LCAOMO-SCF procedure for all valence electrons, has advantage not only in taking account of the effects of the polarization of u core, but also in dealing with the x* u and u* x transitions. Recently Sprinkel et a1.I' have indicated the necessity of including the a*-u states and interpreted that the main contribution to the Faraday B terms of the lowest a* x transition of indole comes from the magnetic coupling of the x*-u states around 5oooO cm-l with the lowest a*-astate. The Faraday B terms of the vibronically induced a* n transition in formaldehydeIoaand the allowed IAzu 'AI, (a* a) transition in benzenei2have also been calculated using wave functions obtained from the CNDO approximation. On the other hand, the electronic structures of pyridine, diazines, and 1,3,5-triazine have been investigated with the aid
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Kaito, Hatano, Tajiri
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/
Faraday B Terms of Some Azaheterocycles
5242
MCD
x IO
{ 0.02 -0.02 -0.04
&i$znp
pyridrztne
C2"
C2"
pyrrzine
5-triazine
b
K*-N STATE
l
benzene
4h
D3h
'2,
E
A!:
f
A2
t
I / l
ux I
O - ~ ( ~ ~ - I
Figure 2. MCD and UV spectra of pyridine in n-heptane solution at room
temperature. pyridazine pyr,m,dlns
pyridin
pyrazine
C2"
C2"
s-lriazine
'Zh
D3h
Figure 1. (a) Four lowest T*-T states of benzene, pyridine, and azines. (b)
r*-n states of pyridine and azines arising from electron promotion from the lone pair orbital of nitrogen atoms to the lowest vacant ?r orbital. Solid and dashed vertical lines denote the allowed and forbidden transitions, respectively. Table 1. Atomic Parameters (eV) Used in the Present Calculations
us, C N H K
UPP
-50.686 -70.093 -13.593
-41.530 -57.848
YAA
yrr
10.207 11.052 12.848
10.93 11.88
P 9.88 10.47
-17.5 -24.4 -5.9
= 0.6
of the all valence-electron molecular orbital theory.14J5 The electronic states expected in the lower energy region are presented in Figure 1. Four lowest a*-a states are related to the ~Bz,,,~BI,,,and lElu states of benzene. However, in pyridine and diazines, the degeneracy of the lElustate is removed and two lowest a* a transitions become electrically allowed because of the symmetry reduction. The presence of the lone pair on nitrogen atom causes the possibility of a* n transitions, one of which is electrically allowed (see Figure 1b), and is observed at lower wavenumber than the first a* a transition. Because of these closely spaced a* n and a* x transitions, these azaheterocycles are considered to be suitable materials for the theoretical MCD work including the a* u type transitions which are polarized along the axis perpendicular to the molecular plane. In the present article, we apply the CNDO/S-CI method to the calculation of the B terms of pyridine, diazines, and 1,3,5-triazine, aiming at elucidating the perturbing mechanism for the Faraday B terms of the a* n and a* a transitions of these compounds.
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Experimental Section
duced pressure. Pyrazine (Wako Pure Chemical Industries, Ltd.) was recrystallized from petroleum ether and finally dried in vacuo. Under reduced pressure 1,3,5-triazine (Tokyo Chemical Industry Co., Ltd.) was twice sublimed. All measurements were performed at room temperature using spectrograde n-heptane as a solvent. The MCD spectra were recorded with a JASCO J-20A recording circular dichrometer equipped with a 11.4 kG electromagnet. The MCD spectra were calibrated with freshly prepared potassium ferricyanide, [e], at 4220 8, = 1.0' cm2 mol-' (3-'. The UV spectra were measured on a Hitachi EPS-3T recording spectrophotometer.
Theoretical Section The quantum mechanical model used in this work is principally based on the CNDO/S-CI method proposed by Pople, Santry, and Segal.13 However, some modifications were made along the lines of the CNDO/S-CI method proposed by Del Bene and Jaff6.I4 One-center electron repulsion integrals, YAA, and one-center core parameters, Us,and Up,, in the HartreeFock matrix elements were taken from the values determined by Sichel and Whitehead.16 Two-center electron-repulsion integrals were calculated using the Nishimoto-Mataga equation.]' The values of the bonding parameter, @, and the empirical parameter, K , were adjusted so that the ionization potentials and the transition energies of benzene, pyridine, and hydrogen molecule are well reproduced. Configuration interaction (CI) among singly excited configurations below 10 eV was taken into account. The electron repulsion integrals in the a*-a CI matrix elements, ytr, were set to be equal to ypp,and those in the T * - U and u*-a C I matrix elements, Y ~ were ~ ,assumed to be (yps 2yp,
