9208
J. Phys. Chem. 1991, 95, 9208-9210
the 2008-cm-' band in I8O isotopic experiments can provide conclusive evidence for the equilibrium structure of sym-NO,.
in the completion of this project. This work was supported in part by NASA Grant NGT-70200 and the Pittsburgh Supercomputing Center Grant ATM900005P.
Acknowledgment. We thank the authors of refs 4 and 12 for timely discussions and for making data available prior to publication. We also thank Dr. Razi Hassan for his generous support
Registry No. NO,, 12033-49-7; "No,, 136705-21-0; N 1 * 0 0 2 , 136705-22-1 ; V0I8O,, 136705-23-2; N"03, 136705-24-3; "N, 1439096-6: I8O. 14797-71-8
Torsional Angles in Alkenylarenes Studied by Semiempirical Molecular Orbital Methods Yong Ni, Jay S. Siegel, and David R. Kearns* Department of Chemistry, University of California, S a n Diego, La Jolla, California 92093-0342 (Received: September 17, 1990; In Final Form: April 11, 1991)
Equilibrium torsional angles and torsional potentials for 1- and 2-vinylnaphthalenes, trans-, cis-, and gem- I-naphthylpropenes, and 2- and 9-vinylanthraceneswere calculated by using the MNDO and AM1 Hamiltonians. The AMI and MNDO results for the above compounds are compared with experimental findings in the literature.
Introduction Conformational problems are frequently encountered in the interpretation of the electronic spectra of organic compounds.l We have observed that quinacrine and the related compound 9-amino-6-chloro-2-methoxyacridine exhibit a complex fluorescence behavior.2 In an effort to develop a mechanism that could reasonably account for the experimental observations, we have performed semiempirical molecular orbital calculations on a series of alkenylarenes.) Recently, Facchine et aL4 determined the torsional angles between the vinyl and aryl groups in 1- and 2-vinylnaphthalene (1 VN and 2VN) by the N M R quadrupolar splitting method. Anderson et ale5measured these torsional angles in 1VN and its methyl derivatives, trans-, cis-, and gem- 1-naphthylpropene (I-NP, c-NP, and g-NP) by the nuclear Overhauser effect (nOe) method. We have reported6 the torsional angle determined by the N M R quadrupolar splitting method and AM1 computations for 2vinylanthracene (2VA). Werst et al.' studied the vibrational structure of jet-cooled 9-vinylanthracene (9VA) by laser-induced fluorescence methods and deduced the torsional angle. Given the wealth of experimental findings, we found it worthwhile to compare the performance of the M N D 0 8 and AM19 semiempirical molecular orbital methods on the above compounds. These calculations help to assess the applicability and limitations of these two methods to conformational problems. In this conformational study, the key structural feature of the above hydrocarbons is the torsional angle 8 between the alkene and aromatic ring planes. To remain consistent with the experimental data in the literature;ss the structures in Figure 1 are taken as the 8 = 0' conformations, Le., syn for 2VN and 2VA and anti for 1VN and its methyl derivatives. Methods All molecular orbital calculations were performed on a Sun 3/ 140M-4 workstation with floating point accelerator using the ( I ) Jaffe, H.H.; Orchin, M. Theory and Applicafion of Ulrrauiole! Specfroscopy; Wiley: New York, 1962; pp 384-449. (2) Fan, P.; Hird, T.; Kcarns, D. R. J. Phys. Chem. 1989,93,6615-6622. (3) Ni, Y.;Kearns, D. R. J. Phys. Chem. 1989, 93, 6622-662s. (4) Facchine, K. L.; Staley, S. W.; van Zijl, P. C. M.; Mishra, P. K.: Bothner-By, A. A. J. A m . Chem. Soc. 1988, 110, 4900-4905. (5) Anderson, J. E.; Barkel, D. J. D.; Parkin, J. E. J . Chem. SOC.,Perkin Trans. 2 1987, 955-959. (6) Ni, Y.; Siegel, J . S.; Hsu, V. L.; Kearns, D. R. J . Phys. Chem., following paper in this issue. (7) Werst. D. W.; Brearly, A. M.; Gentry, W. R.; Barbara, P.F. J . Am. Chem. Soc. 1987, 109. 32-40. (8) Dewar, M. J. S.; Thiel, W. J . A m . Chem. SOC.1977, 99,4899-4907. (9) Dewar, M. J. S.;Zoebisch, E. G . ;Healy, E. F.; Stewart, J. J. P. J . Am. Chem. SOC.1985, 107. 3902-3909.
MOPAC program packagelo equipped with M N D O and AMI options. The aromatic ring and the vinyl group were individually constrained to remain planar; no other geometrical constraints were imposed. This constraint was not imposed on H a and Hortho. As a starting point in the calculations, we assumed a planar geometry. The torsional potential for alkenyl rotation was computed in steps of 10' (for compounds 1,2, and 6 and 7) and 15' (for compounds 3-5) with optimization of all remaining geometrical parameters. The following was reported and used for the potential energy diagram.
AE = E(8) - E(0') Additional p i n t s were then computed about the energy minima in order to better define the torsional potential surface at each minimum.
Results and Discussion The torsional potentials of the above compounds calculated by AM1 and MNDO are shown in Figures 2-8. The principal values on the diagrams along with the experimental mean angles from literature are collected in Table 1. MNDO and AMI. From the results listed in Table I, it is clear that MNDO, which consistently and incorrectly predicts the perpendicular conformation as the most stable one, is not appropriate for conformational studies of these compounds. On the contrary, the AM1 results are in good agreement with the experimental measurements except in the case of 9VA (vide infra). Fabian has reported similar conclusions in his comparison of MNDO and AM1 to experiment in the conformational study of pheny1pyrroles.l When comparing the calculational results with the experimental data, it should be borne in mind that the derivation of the angle is method dependent. The angle in the AM1 method denotes the angle value corresponding to the potential energy minimum, whereas the quadrupolar splitting method yields directly (sin2 8 ) and the nOe method yields (rHH3)-2which is related to 8 by trigonometric relation^.^,^ For deep, well-defined minima these methods will arrive at the same value, whereas for shallow, asymmetric minima they may diverge. In our present case AM1 predicts a relatively shallow potential energy minima, and therefore the excellent agreement between experiment and theory is surprising. However, this may stem from the fact that AM1 calculations are known to give accurate minima but too shallow potential surfaces for vinylaromatic systems such as styrene.12 (IO) Stewart, J. J. P. QCPE Bull. No. 455 (Version 4.00); released in 1987. ( I I ) Fabian, W. Z.Nafurforsch. 1987, 42A, 641-644.
0022-3654/91/2095-9208%02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9209
Torsional Angles in Alkenylarenes
6 1
W
& \
/
III
-6
0
m /
/
20
40
60
120
100
60
140 160
180
Torsional Angles (Degrees) Figure 4. Torsional potential for rotation of the propenyl group of t-NP calculated by AM1 (open squares) and MNDO (filled diamonds). ''I
Figure 1. Structures of 2VN, IVN, t-NP, c-NP, g-NP, 2VA, and 9VA.
W
0
W
20
40
80 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0
60
Torsional Angles (Degrees) Figure 5. Torsional potential for rotation of the propenyl group of c-NP calculated by AM1 (open squares) and MNDO (filled diamonds). - 2 ! .
0
, . , . , . , 20
40
60
60
. , 100
. , . , . , 120
140 160
.
21
1
160
Torsional Angles (Degrees) Figure 2. Torsional potentials for rotation of the vinyl group of 2VN calculated by AMI (open squares) and MNDO (filled diamonds).
-
- l S - I . v . , . . , . 1 0 20 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 I
W
Torsional Angles (Degrees) Figure 3. Torsional potentials for rotation of the vinyl group of 1VN calculated by AM1 (open squares) and MNDO (filled diamonds). MMP2 and AMI. An empirical force field program MMP2I3 was useds to account for the experimental results of 1VN and its
.
,
.
,
.
I
.
,
Torsional Angles (Degrees) Figure 6. Torsional potential for rotation of the propenyl group of g-NP calculated by AM1 (open squares) and MNDO (filled diamonds). methyl derivatives. In the case of g-NP, however, there was some difficulty in reconciling the experimental mean angle (1 14O) with the calculated minima by MMP2$ (49" and 1 3 1 O ) . In contrast, AM1 predicts torsional angles in excellent agreement with the experimental findings for all of the above compounds including g-NP. 9-Vinylanthracene. Werst et al.' reported the laser-induced fluorescence spectra of 9VA and determined the best-fit torsional potential in the first excited singlet state (S,)and a relative phase (12)
(13)
Fabian, W. M.F. J. Compuf. Chem. 1988, 9, 369. Allinger, N. L.; Flanapn, H.L. J. Compur. Chem. 1983,4.399-403.
9210 The Journal of Physical Chemistry, Vol. 95, No. 23, I991
Ni et al.
TABLE I: TorsionaI Angles, 0 ( d e ) , at Energy Minima and Their Relative Energies A E ( A E = E ( 0 ) - E(Oo) in kcal/mol) AE 6 = 90° B = 180°
8, A E in other
method
6
AE
stable conformations
2VN
AM I MNDO
19 88
-0.40 -1.36
160, 0.10
IVN
AM 1 MNDO
40 83
-1
.oo
121, -0.12
AMI MNDO
40 83
-1.03 -4.23
122, -0.06
C-NP
AMI MNDO
50 85
-3.08 -7.15
108, 219
g-NP
AMI MNDO
115 92
-9.28 -17.03
2VA
AM 1 MNDO
1 8c 84
9VA
AM 1 MNDO
65 90
t-NP
1.49
0.19 0.25
18.3 f 3.1"
- I .36
-0.02 -3.89
4.94 4.28
39.8 f 2.1'or 43.8 f 2.0" 38b
0.10
5.23 4.78
436
-2.26 -7.12
10.05
65 f 5b
74, 9.06
-9.14 17.02
0.77 0.53
159, 0.21
I .53 -1.30
0.27 0.34
-3.93
-4.17
0.04 -1.31
exptl mean angle, den
9.13
9.46
-9.68 -13.64
1 14b
17.3 f 3.4c
od
0.0 0.0
-1 3.64
Reference 6 (NMR quadrupolar splitting method).
Reference 4 (NMR quadrupolar splitting method). bReference 5 (nOe method). dReference 7 (laser-induced fluorescence method).
-
;
I
m
5. W
-12
-2! 0
'
'
I
20
1
40
'
I
60
Torsional
'
u
'
I
.
1
.
I
'
I
'
1
80 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 Angles
(Degrees)
.
-14 0
I
20
.
I
40
. 60
Torslonal
80
1 0 0 1 2 0 1 4 0 1 6 0 180
Angles
(Degrees)
Figure 7. Torsional potential for rotation of the vinyl group of 2VA calculated by AMI (open squares) and MNDO (filled diamonds).
Figure 8. Torsional potential for rotation of the vinyl group of 9VA calculated by AM1 (open squares) and MNDO (filled diamonds).
factor of 0' between the ground electronic state (So) and SI. In the case of 9VA there was no structurally similar molecule for which the SI geometry was known, so they felt that their steric evaluation was somewhat less certain. Finally, torsional angles of 0' in S,and So were chosen to account for the experimental observation. Our A M I calculation gives an equilibrium torsional angle of 65' and a barrier at 90' of only 0.2 kcal/mol. The values for the quadrupolar splitting at Oo and 90' are similar in magnitude but opposite in sign. The sign of the splitting is difficult to assess. Therefore, in the absence of additional experimental or computational data, it is difficult to distinguish between these two extremes by this N M R method. Anderson et ala5measured the torsional angle in IVN to be 38' by the nOe method (vide supra). If one compares 9VA and IVN
on the basis of sterics, there is more repulsion to the vinyl group in 9VA than in 1VN because of the neighboring H atom on the additional benzene ring in 9VA, and the torsional angle in 9VA should therefore be greater than that in IVN. Rather than a planar conformation, it may be more appropriate to assign a near-perpendicular conformation for 9VA, in order to explain the experimental observation of Werst et al.'
Acknowledgment. This work was supported by grants from the American Cancer Society (ACrS CH-32L) and the National Science Foundation (DMB86-04545) (D.R.K.) and American Cancer Society Jr. Faculty Fellow (C-58024) and the N S F PYI program (CHE-8857812) (J.S.S.). Registry No. IVN, 826-74-4; 2VN, 827-54-3; r-NP, 53269-01-5; C-NP. 53269-00-4; g-NP, 1855-47-6; 2VA, 2026-16-6; 9VA, 2444-68-0.
