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Rotational isomerism in 2-methyl-1,2-dinitropropane
Journal of Molecular Structure, 221 (1990) 115-126 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
115
ROTATIONAL ISOMERISM IN 2-METHYL-1,2DINITROPROPANE Y.L. LAM, B.G. TAN and H.H.HUANG* Chemistry Department, National University of Singapore, Lower Kent Ridge Road, 0511 (Singapore) (Received 9 June 1989)
ABSTRACT IR and Raman spectra of 2-methyl-1,2_dinitropropane in the solid and solution states are reported and assignment of frequencies is made. Comparison of the Raman and IR spectra in both the solid and solution states indicates that the compound exists in the polar gauche conformation in the solid state and as a rotameric mixture of the gauche and non-polar trans conformers in solution. Analysis of dipole moment data shows that in carbon tetrachloride at 25”C, the mixture contains approximately 12% trans and 88% polar gauche rotamers. In benzene solution, the gauche population is further enhanced. These experimental results are compared with those obtained from a semiempirical MO programme AMPAC (AM1 ) calculation.
INTRODUCTION
Our recent studies on rotational isomerism in nitro compounds have thus far been carried out on the symmetrically substituted dinitroethanes [l-4]. This paper reports our findings on a related molecule of lower symmetry, 2methyl-1,2dinitropropane, based on dielectric, electric birefringence, IR and Raman spectroscopic measurements. A semiempirical molecular orbital study of the molecule has also been undertaken to estimate the relative energies and geometries of the gauche and tram rotamers. EXPERIMENTAL
Solute 2-Methyl-1,2dinitropropane was prepared by the reaction of isobutylene with dinitrogen tetroxide according to the method of Levy et al. [5]. It had m.p. 53-54°C. Elemental analysis: found, C 32.14, H 5.40, N 18.65; talc. for C4H8N204, C 32.43, H 5.40, N 18.91. *Author to whom correspondence should be addressed.
0022-2860/90/$03.50
0 1990 Elsevier Science Publishers B.V.
116
Solvents All solvents were carefully distilled and stored over drying agents before use. The physical constants required in the dielectric and Kerr effect measurements have been given previously [ 6,7 1. Apparatus and computations Kerr constants were measured photometrically [ 81 while the dielectric constants were determined with a heterodyne-beat meter [ 91. Densities and refractive indices were measured by standard procedures [lo]. Solid-state IR spectra were recorded as Nujol and hexachlorobutadiene mulls. The solution-state spectra were obtained using solvents such as carbon tetrachloride, carbon disulphide, benzene, chloroform and acetonitrile. The concentrations of the various solutions were expressed in terms of weight percentages. The numbers given in parentheses after the wavenumbers indicate the absorption peak heights, and hence the approximate relative intensities of the different bands. Solid and solution-state Raman measurements were made using the 4880 A line of a Spectraphysic argon-ion laser and a JESCO NRlOOO spectrometer. Semiempirical molecular orbital calculations were performed using the program AMPAC [ 111. AM1 parametrization [ 121 was used and full geometry optimization was performed for each incremental value of the ethane C-C torsion angle. In the calculation, advantage was taken of the published geometry of a related molecule, 2,3-dimethyl-2,3-dinitrobutane [ 131. Calculations were performed on a VAX 8650 computer using FORTRAN 77 operating under VMS. Torsion or dihedral angles (8) were defined by the atoms >N-C-C-N< in 2methyl-1,2dinitropropane by the convention of Klyne and Prelog [ 141. RESULTS AND DISCUSSION
The results of all the physical measurements are presented in Tables l-3 with standard notation. Figure 1 shows the Newman projections of the likely rotamers of the compound where (2) and (3) are polar mirror images of each other. Spectroscopy For the trans rotamer (C, symmetry), 48 fundamental modes can be expected as given by 3N- 6 (where N= 18 is the number of atoms in the molecule). Twenty six of these fundamental modes are associated with class A’ and 22 with class A”. All the absorptions are expected to be both IR and Raman active as the molecule lacks a centre of symmetry. The gauche rotamer with C,
117
symmetry would have 48 A-type fundamentals which are both IR and Raman active. Comparison of the IR and Raman spectra of the compound in the solid state shows that many absorption bands coincide in frequency. This is to be expected since the conformations in which the molecule can exist do not possess any centre of symmetry. However, the presence of an extra IR band at 442 cm-’ and an extra Raman band at 2940 cm-’ in the spectra of the compound in the solution state suggest strongly that at least two rotamers are present in solution while only one rotamer is present in the solid state. The band at 442 cm-’ was found to decrease in intensity with increasing polarity of the solvent and may thus be attributed to the non-polar rotamer (1). Hence, the polar rotamer (2) or (3) where the nitro groups are gauche to each other appears to be the stable form in the solid. This is consistent with most of the dinitro compounds studied so far [3,13]. 2-Methyl-1,2-dinitropropane differs from 2,3-dimethyl-2,3dinitrobutane only in having two hydrogen atoms in place of two methyl groups. In making spectral assignments, advantage has been taken of our earlier study on the latter compound together with papers on its bromo and chloro analogues [ 15,161 and on isobutane [ 171. Comparing the spectra of 2-methyl-1,2_dinitropropane with those of 2,3-dimethyl-2,3dinitrobutane, extra bands are found at 908, 1275,2470,2750 and 2855 cm-‘. The band at 908 cm-l with its characteristic strong intensity may be identified with the CH, rocking modes while the absorption at 1275 cm-’ can be attributed to the CH, twisting and wagging vibrations. The bands at 2470 and 2750 cm-l may arise from combination bands due to the C-H bending modes while the band at 2855 cm-’ is probably due to the symmetric CHz stretching vibrations. The bands associated with the C-C stretching and CH, rocking vibrations are expected in the spectral region 1250-710 cm-‘. Coupling between each of the C-C vibrations in the skeleton together with the coupling of the C-C stretching modes with the CH, rocking modes make the assignments rather difficult. The C-NO, stretching modes are known to give rise to strong absorption in the 1560-1340 cm-l region [ 181. This result is confirmed particularly by the Raman data. The bands at 1565 and 1354 cm-’ of the IR spectra and at 1555 and 1345 cm-’ of the Raman spectra are assigned to the respective antisymmetric and symmetric C-NO, stretchings. Diallo [ 19 ] has attributed the absorptions in the range 1060-850 cm-’ for dihalogeno-dinitromethanes to C-N stretchings. Similarly, the two relatively strong bands between 865 and 835 cm-l have been assigned to the C-N stretching. This assignment is reasonable because isobutane and the dihalogeno derivatives of this compound (in which C-N bonds are absent) do not have any strong absorptions in this region. The broad bands in the 650-610 cm-l region have been assigned to the bending modes of the nitro group. The bands in this region could also be assigned to
3030(1.5) 3000(7) 2950(l) 2925(8.5) 2890(2) 2855 (0.75)
d
3030(7) E
1525sh ( 1) 1472(14) 1460(3) 1445(4.25) 1430(6.25) 1405(20) 1378(37)
d
1460(17) 1455(0.5) 1440(0.5) 1426(7) 1407 (20)
1560br(47)
d
d
d
d
d
2750(3.75) 2695(5.25) 2470(3.75) 2405(3.25) 1565br(38)
c
c
HB
Nujol
2739(7.75) 2680(6.75) 2465(5.5) 2395(6.75) 1580(76) 1570sh(5) 1528(36) 1470(14) 1456(16) 1440(13) 1417(9.75) 1402(18) 1375(40) 1398(32) 1370(57)
e
2735(3.5) 2680(2.75) 2470(1.75) 2395(4) E
1440(17) 1418(17.5) 1395 (17.5) 1385(63)
c
2740(7.75) 2685 (6.75) 2465 (2) 2405(8.5) 1570(88) 1555(2) e
d
1470( 13) 1458(2.25) 1440(4) 1424(1.5) 1405(11) 1375(35)
d
d
2740(5) 2685 (3.5) 2465(3.5) 2410(3.5) 1565br(48)
d
d
d
2930(10) 2890 (9 )
2750(3.5) 2690(2.5) e e e e e e c e e e e
d
c
c
c
e
2990(19) 2950(22) 2930(7) 2890( 13)
c e
e
c
e
3020(4) 2995 (33.25) 2945(12.25) 2930 (2.25) 2895(9.5)
CH,CN (3.0 wt.%) 37.P
3020(4.75) 2995(55) 2945(19) 2930(8) 2890(17)
CHCl, (3.0 wt.%) 4.806b
GH, (1.9wt.%) 2.284b
CR, (1.1 wt.%) 2.641b
cc14 (3.0 wt.%) 2.23ab
IR spectrum* of 2-methyl-1,2_dinitropropane
TABLE 1
i
i
SC& ( sym )
vNOz (asym)
Combination
vCI-Is(sym 1
uCH, (asym ) & VCH,(sym)
Assignment
530(12.75) 459(10.25)
d
e
836(3) 752(7.25) 708(5.75) 636(l) 612br(2) 575br( 12) 530(3) 455(5.25)
907 (5.75)
650( 14) 633br(12.5) 586(3.75) 527(25.5) 458(7.5) 442(3)
1240(25.5) 1182(17) 1146(19.75) 1023(10) 990(2.75) 949 (8.5) 933(19) 904(35.25) 858(61) e c c
1345 (25.75) 1271(44) 1242 (25) 1181(8.25) 1147(9) 1023 (4.75) 996(2.5) 949(3.25) 933(14) 904(21.5) 858(41.25) 830( 16.5) 755(23) 708( 12) 651(U) 625br(3) 586(2) 528(13) 465(l) 445(3)
1346(49.75) 1273 (32)
528(18.75) 464(4) 449(3.5)
e E e c c
933(15) 906(25.5) 860(41.25) 830(10)
1239(31) c c c c c
1348(44.75) 1273(43)
d
e c c
445(6)
630br(12) 586( 1.5) 528( 17)
996(5.25) 950(7.25) 930(8) 904(22.5) 860(39) 830(8.25)
1148(34) e
c c
1350(13) 1273 (37)
950(3) 935(3.75) 903(5.5) 865(46.5) 835( 19.5) 760(27) 710(19.75) 640sh(1.5) 620(6.5) 580(0.5) 529(7.5) 465(4) 444(3)
1250(22.5) 1185(13) 1155(3) e e
1275 (40)
c
LX
PNC,
UC-C,PCH,
vCN
pCH, (rock)
PCH,, UC-C
JM&(sym)
CH2 (twist and wag)
“Absorption peak heights (and hence approximate relative intensities) are given in parentheses. “ezOvalue. “Masked by solvent. dAbsence of absorption.
d
d
1247 (8) c
1245(6.75) 1185(17) 1155(9) 1030(11.25) lOOO(5) 953(7.75) 937(17.75) 908(21.75) 865 (27.75) 835 (19.75) 752(22.25) 710(17.75) 636(3) 613br(8.5)
1151(l) 1029(2.75) e e e
1351(23.75) 1275(12)
1354(19) 1275(19)
120 TABLE 2 Raman spectrum” of 2-methyl-1,2-dinitropropane Solid
CHCl, (35 wt.%) 4.806b
CsHs (40 wt.%) 2.284b
CH,CN (61 wt.%) 37.5b
CCI, (36 wt.%) 2.238b
CS, (40 wt.%) 2.641b
3010(15) 2970(16.5) 2955(20.5) d
c 2970(18) 2950(25.5)
3010(13) ’ c
= e e
3002(17.5) 2975(24.5) 2955(32)
3005(12.5) 2980(18) 2955(26)
2875 (6)
2940(18) 2875(5.5)
2940(21) 2875(4)
c ’
2930(24.5) 2880(10.5)
2940(18) 2870(4)
2745(5) 1555( 17) 1455(18) 1440(18) 1420(16)
2740(3.5) 1555(17) 1455(16.5) 1440(15) 1420(15)
2750(3.5) 1560(20) 1460(13) 1442(13) ’
’ 1557(22.5) ’ ’ c
2740(8) 1555(19.5) 1455(18.5) 1445(20) 1420(19)
2740(2) 1555(15) 1460(14) 1445 (16.5) 1420( 16)
1400(23.5) 1375(46) 1345 (35) 1270(8) 1240(7.5) 1180(8) 1145(7) 1025(6.5)
1400(30) 1372(53)
’ 1375(57)
1402(33.5) 1372(61)
1345(36) 1275(8.5) 1240(10) 1182(11) 1137(8) 1027(6)
1348(36) 1272(7.5) 1245(6.5) ’ 1150(13) ’
1405(37) c 1350(41) 1280(8.5) 1245(11) 1182(12) 1150(11) e
1345(43.5) 1275(10.5) 1240(9) 1180(11.5) 1150(10) 1025(8)
1402(32) 1375(58.5) 1347(40.5) 1270(7) 1235(6) 1180(7.5) 1150(5) 1027(4)
950(14.5) 937(20)
lOOO(13) 952(18) c
995(15) 950(11.5) 935(24.5)
1000(10) 950(14) 935(20.5)
907(36)
’
905(47.5) SSO(26) 835(100)
907(43) 860(19) 835(100)
995(12) 950(14.5) 935(17)
995(11.5) 950(13) 935(21)
c
905(33) 860(17.5) 835 (80.5)
905 (38) 860(19) 835 (96.5)
’
802(g) 755(14) 710(8.5)
800(13.5)
’
c
637(11.5) 612(13) 580(23) 555(25.5) 525(29) 440(19.5) 365(40) 335(30) 265(33)
625(12.5) 610(12.5) 580(26.5) 560(27) 530(31.5) 442(20) = 335(31.5) ’
865(20) 835 (94) 802(11)
835 (94.5) 760(15) 710(7.5)
710(12) ’ ’
585(27.5) 560(27) 530(30) 440(19) 370(42) 337(27) 267(26.5)
’ d
’ c
’ 337(30) 270(27)
755(17) 710(7)
710(U)
710(10) 610(13) 580(35) 560(35) 535(35) 445(24)
c
637(14) 610(16) 580(37) 560(37) 527(40.5) ’ 365(48) 332(40) 265(35)
’
Assignment
~CH,(asym),~CH,(~ym)
vCH,(sym)
i
Combination vNO, (asym) GCHa(=ym)JCH,
=H,(sym)
i
vN0, (sym ) TCH, (twist and wag)
PCHQC-c
pCJ%hC
1 PCHZ YCN > vc-c,pCH,
617(14) 580(30) 555 (30.5) 530(37.5)
PNO,
447(22) 365(46.5) 335(30) 265(29)
> PC-C-C
“Absorption peak heights (and hence approximate relative intensities) are given in parentheses. bt~~value. ‘Masked by solvent. dAbsence of absorption.
the C-C vibrational modes of the central and non-central carbon atoms. In addition, coupling between the NO, bending modes and the skeletal deformation modes may occur.
121 TABLE 3 Polarizations, dinitropropane’
refractions,
Temp. (“C)
Solvent
7 25 45 7 25 45 25
Benzene Benzene Benzene ccl, ccl, ccl, Cyclohexane
dipole momenta
Cont. range ( l@w, )
(Ye,
520-1390 830-5120 469-1891 540- 900 400- 800 300- 870 200- 550
17.79 15.69 14.11 27.90 24.77 22.17 10.62
and molar Kerr constants
/I
0.376 0.318 0.362 -0.274 -0.265 -0.249 0.356
Y
6
-0.026
-5.48
-0.018
-40.33
at infinite
dilution
of 2-methyl-1,2-
(cm3) (ems)
1o,,p (Cm)
lo27 (mK,) (m5V-2mol-1)
508.2 471.6 442.7 459.2 422.8 397.0 408.7
15.59 15.45 15.41 14.76 14.56 14.53 14.28
Pz
Rn
30.8
31.9 32.4
- 151
-75
“Incremental changes in the relative permittivites, densities, refractive indices and Kerr constants (de, Ad, An and AB respectively) were measured for solutions having solute weight fractions q. The coefficients (Y,/?, y and Gwere derived from the relations (it,= IA~JI&II,,~~,= EAd/&a,, ynf = XAn2/Ew, and 6B,= ~AB/Iw,. %alculated on the basis that Dp= 1.05Ro.
H
NO2 (1) trans
(2) gauche
Ii (3) gauche
Fig. 1. Newman projections of 2-methyl-1,2-dinitropropane.
Comparison of solution-state spectra
Although all the bands of the solid may be attributed to the gauche rotamer, it is interesting to note that many of these bands decrease in intensity on dissolution in more polar solvents. These bands may be due to the extra trans bands which appear in solution, coinciding with the gauche bands. Such bands attributable to a mixture of gauche and trans rotamers are at 2995,2739,2680, 2395,1580,1456,1440, 1417,1402,1375, 1345, 1271,933,904 and 650 cm-l. Besides these bands, there are a few bands which decrease in intensity on dissolution in the non-polar solvents but increase in intensity with increasing
122
polarity of the solvent. These are the purely gauche bands which occur at 2945, 835,755 and 708 cm-‘. The intensities of these gauche bands however do not vary very significantly with changes in the polarity of the solvents. This implies that there is not much change in the population of gauche rotamer on changing the solvent, probably because the population of the gauche rotamer is already close to saturation. Hence from the IR and Raman spectra of the compound, we conclude that 2-methyl-1,2dinitropropane exists exclusively in the gauche conformation in the solid state, while in solution a mixture of gauche and truns rotamers is present with the truns rotamer increasing in proportion as the polarity of the solvent decreases. Dipole moment and Kerr effect measurements Results of the dipole moment measurements in benzene, carbon tetrachloride and cyclohexane at various temperatures are summarized in Table 3. Kerr effect measurements in benzene and carbon tetrachloride are also included. Table 3 shows that the dipole moments of 2-methyl-1,2dinitropropane in carbon tetrachloride and cyclohexane are very similar and hence the average of these values may be considered to be the dipole moment of the compound in the absence of solvent/solute interaction. The large dipole moment means that the polar rotamer (2) or (3) must be present in high proportion in these solvents. Table 3 also shows that the dipole moment of the compound decreases with increasing temperature in carbon tetrachloride. It follows from Boltzmann statistics that the gauche rotamer is more stable than the truns, and is greater in population in this solvent. Analysis of our dipole moment data in Ccl, according to the Lennard-Jones-Pike method [20] gives an energy difference (A& =Ep -Et) between gauche and truns of -7.172 kJ mol-l. The dipole moment of the gauche rotamer (Pi) obtained from this analysis was 15.49 x 10d3’ Cm. The dihedral angle of the gauche rotamer may be calculated from its dipole moment as follows: the resultant moment p (0) of any rotamer of a substituted ethane XCR,-CR2 X is given by (I) where pi and b are the moments of the (C&X) and (C&X) moieties of the molecule respectively; (Y~and cry2 are the supplementary angles of the central C (1 )-C (2)-X and C (2)-C (1 )-X bond angles respectively and 8 is the dihedral angle between the two C-C-X planes. In these calculations, ,ul was taken to be the value of the dipole moment of 2-nitropropane (11.74~10e3’ Cm) [ 11, p2 the moment of nitromethane (10.45 x 10e3’ Cm) [ 211; from the optimization of the geometry of 2-methyl-1,2dinitropropane, czl and (Yewere found
123
to be 70.3” and 64” respectively, using AMY Since asp= 15.49 X 10P3’ Cm, substitution of these values in eqn. (1) gives 8= 81.6”. These values of AE, and 8 correspond to a gauche rotamer population of 88% at 25°C. The very high proportion of the gauche rotamer suggests that in this compound the gauche conformation is inherently much more stable than the ~M~LS,despite the expectation that the two bulky nitro groups would tend to avoid each other to the maximum extent by adopting the tram conformation. This stability appears to be another manifestation of the “guuche effect” noted by Wolfe [ 221; it is consistent with the theory that there is a tendency for that structure to be favoured which has the maximum number of gauche interactions between the adjacent electron pairs and/or polar bands. In 2-methyl-1,2dinitropropane, each nitro group has a very high polarity of ca. 11.68 x 10w3’ cm and possesses two electron pairs on each oxygen atom and one electron pair on the nitrogen atom. However the final conformation adopted must be decided by the overall balance of attraction and repulsion between all the groups in the molecule. The variation of the heat of formation obtained from AM1 calculations with the NO,-C-C-NO, dihedral angle is given in Fig. 2. Clearly the gauche rotamers have a lower energy than the trum form. The energy difference between theguuche and truns forms obtained from the calculations was - 6.53 kJ mol-’ and the torsion angle of the gauche rotamer of lowest energy was found to be 66” when the geometry was fully optimized. A Gaussian 80 (3-21G) calculation on the gauche rotamer with full geometry optimization gave a torsion angle of 69”. From the Boltzmann distribution, the relative populations of the 2-METHYL-1.2-DINITROPROPANE
-55
1
-80
( 0
I
30
I
60
I
90
I
1
120
150
TORSIONAL
I
160
ANGLE
I
210
I
240
I
270
I
I
300
330
r 360
(DEGREES)
Fig. 2. Energy of 2-methyl-1,2-dinitropropane
as a function of the N-C-C-N
torsion angle.
124
two rotamers separated by an energy difference of -6.53 kJ mole1 would be 4% for the trans state and 96% for the gauche state. This trans/gau.che population ratio is in reasonable agreement with the experimentally derived populations. The difference of some 15” between the calculated torsion angle and the experimentally derived value is not entirely unexpected as the AMPAC and Gaussian 80 values represent the situation in vacua which is likely to be modified in solution. From Table 3, it can be seen that the dipole moment of 2-methyl-1,2dinitropropane in benzene also decreases with increasing temperature. From the experimental data, the internal energy difference between the gauche and trans rotamers was found to be - 8.672 kJ mol-‘. This corresponds to a population of about 94% gauche and 6% trurzs rotamer in solution. The dipole moment of the gauche rotamer was found to be 15.95 x 10m3’ Cm. This yields a dihedral angle of 77” from eqn. (1). In benzene, there is also an augmentation in the dipole moment of 0.89~ 10m3’ Cm relative to carbon tetrachloride at 25°C. Compared to other dinitroethanes studied so far this augmentation is rather small. However, the smallness of the augmentation can be explained because the gauche rotamer is already extensively stabilized. Nevertheless, the existence of a significant difference between the mK values in benzene and carbon tetrachloride suggests that some solvent/solute interaction is present probably in the form of n-complex formation between the “acidic” hydrogens of the solute and the solvent benzene molecules. The negative AmK value ( - 76 x 10z7 m5 Ve2 mol-’ ) further indicates that the resultant dipole moment of the solute is at a large angle to the plane of the solvating benzene molecules as shown below.
Table 4 shows the free energy and enthalpy values for the trans=guuche equilibrium for 2-methyl-1,2-dinitropropane as derived from the variation of dipole moment values with temperature. In both solvents, the difference between m” and AE [MO = A&+ A(PV) ] is positive, implying an increase in volume of the system when the trans molecules are converted to the gauche form under constant pressure. AM1 calculations indicate that although the optimized geometries of the trans and gauche rotamers have similar bond
125
TABLE 4 Thermodynamic quantities governing gauche- trans equilibrium (kJ mol- ’ ) Solvent
Temp. (“C)
K= NJN,
AGO”
ccl,
25 45 7
7.60 7.38 9.87
- 5.026 - 5.285 -5.331
I
Benzene
25 45 7
14.94 13.79 20.89
- 6.699 --7.076 6.939
I
Awb
AE
AHO-AE
-5.215
-7.172
1.957
- 6.908
- 8.672
1.764
“AGOis calculated from the relation AGO= - RTlnK. bmo is obtained from the slope of the 1nK versus l/T plot by the method of least squares and assuming a0 to be constant over the temperature range.
lengths, the C-C-N bond angle of the gauche form is about 3 ’ larger than that of the trans form. This increase in bond angle is larger than the increments found in the other bond angles of the molecule and could probably be due to steric interactions between the bulky nitro groups. Hence the increase in volume when the truns rotamer is converted into the gauche conformation could be due to this increase in bond angle. On the other hand, in benzene solution, the difference between AHo and AE is slightly less positive compared to the corresponding value in Ccl, solution. This is in agreement with our spectroscopic and molar Kerr constant results which show that although the gauche molecules are capable of attracting the solvent benzene molecule to form n complexes more effectively than the trans, the small increase in gauche population on changing the solvent from carbon tetrachloride to benzene would not much affect any physical property due to the presence of the gauche rotamer. ACKNOWLEDGEMENT
We thank Dr. David Winkler for helpful discussions.
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5 6 7
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126 8 9 10 11
12 13 14 15 16 17 18 19 20 21 22
H.H. Huang and SC. Ng, J. Chem. Sot. B, (1968) 582. H.H. Huang and E.P.A. Sullivan, Aust. J. Chem., 21 (1968) 1721. R.J.W. LeFevre, Dipole Moments, 3rd edn., 1953, Methuen, London, Chap. 2; Adv. Phys. Org. Chem., 3 (1965) 1. M.J.S. Dewar, E.G. Zoebisch, E.F. Healeyand J.J.P. Stewart, Q.C.P.E. Bull., 6 (1986) 24. M.J.S. Dewar,E.G. Zoebisch,E.F. Healeyand J.J.P. Stewart, J. Am. Chem. Sot., 107 (1985) 3902. Y. Kai, P. Knochel, S. Kwiatkowski, J.D. Dunitz, J.F.M. Oth, D. Seebach and H.O. Kalinowski, Helv. Chim. Acta, 65 (1982) 137. W. Klyne and V. Prelog, Experientia, 16 (1960) 521. G.A. Crowder and M.T. Richardson, J. Mol. Struct., 78 (1982) 229. M. Hayashi, I. Ichishima, T. Shimanouchi and S.I. Mizushima, Spectrochim. Acta, 10 (1957) 1. R.G. Synder and J.H. Schachtschneider, Spectrochim. Acta, 21 (1965) 169. L.J. Bellamy, The Infrared Spectra of Complex Molecules, Methuen, London, 1954. A.O. Diallo, C.R. Acad. Sci., 272 (1971) 1777. J.E. Lennard-Jones and H.H.M. Pike, Trans. Faraday Sot., 30 (1934) 830. A.E. van Arkel and J.L. Snoek, Trans. Faraday Sot., 30 (1934) 707. S. Wolfe, Act. Chem. Res., 5 (1972) 102.
115
ROTATIONAL ISOMERISM IN 2-METHYL-1,2DINITROPROPANE Y.L. LAM, B.G. TAN and H.H.HUANG* Chemistry Department, National University of Singapore, Lower Kent Ridge Road, 0511 (Singapore) (Received 9 June 1989)
ABSTRACT IR and Raman spectra of 2-methyl-1,2_dinitropropane in the solid and solution states are reported and assignment of frequencies is made. Comparison of the Raman and IR spectra in both the solid and solution states indicates that the compound exists in the polar gauche conformation in the solid state and as a rotameric mixture of the gauche and non-polar trans conformers in solution. Analysis of dipole moment data shows that in carbon tetrachloride at 25”C, the mixture contains approximately 12% trans and 88% polar gauche rotamers. In benzene solution, the gauche population is further enhanced. These experimental results are compared with those obtained from a semiempirical MO programme AMPAC (AM1 ) calculation.
INTRODUCTION
Our recent studies on rotational isomerism in nitro compounds have thus far been carried out on the symmetrically substituted dinitroethanes [l-4]. This paper reports our findings on a related molecule of lower symmetry, 2methyl-1,2dinitropropane, based on dielectric, electric birefringence, IR and Raman spectroscopic measurements. A semiempirical molecular orbital study of the molecule has also been undertaken to estimate the relative energies and geometries of the gauche and tram rotamers. EXPERIMENTAL
Solute 2-Methyl-1,2dinitropropane was prepared by the reaction of isobutylene with dinitrogen tetroxide according to the method of Levy et al. [5]. It had m.p. 53-54°C. Elemental analysis: found, C 32.14, H 5.40, N 18.65; talc. for C4H8N204, C 32.43, H 5.40, N 18.91. *Author to whom correspondence should be addressed.
0022-2860/90/$03.50
0 1990 Elsevier Science Publishers B.V.
116
Solvents All solvents were carefully distilled and stored over drying agents before use. The physical constants required in the dielectric and Kerr effect measurements have been given previously [ 6,7 1. Apparatus and computations Kerr constants were measured photometrically [ 81 while the dielectric constants were determined with a heterodyne-beat meter [ 91. Densities and refractive indices were measured by standard procedures [lo]. Solid-state IR spectra were recorded as Nujol and hexachlorobutadiene mulls. The solution-state spectra were obtained using solvents such as carbon tetrachloride, carbon disulphide, benzene, chloroform and acetonitrile. The concentrations of the various solutions were expressed in terms of weight percentages. The numbers given in parentheses after the wavenumbers indicate the absorption peak heights, and hence the approximate relative intensities of the different bands. Solid and solution-state Raman measurements were made using the 4880 A line of a Spectraphysic argon-ion laser and a JESCO NRlOOO spectrometer. Semiempirical molecular orbital calculations were performed using the program AMPAC [ 111. AM1 parametrization [ 121 was used and full geometry optimization was performed for each incremental value of the ethane C-C torsion angle. In the calculation, advantage was taken of the published geometry of a related molecule, 2,3-dimethyl-2,3-dinitrobutane [ 131. Calculations were performed on a VAX 8650 computer using FORTRAN 77 operating under VMS. Torsion or dihedral angles (8) were defined by the atoms >N-C-C-N< in 2methyl-1,2dinitropropane by the convention of Klyne and Prelog [ 141. RESULTS AND DISCUSSION
The results of all the physical measurements are presented in Tables l-3 with standard notation. Figure 1 shows the Newman projections of the likely rotamers of the compound where (2) and (3) are polar mirror images of each other. Spectroscopy For the trans rotamer (C, symmetry), 48 fundamental modes can be expected as given by 3N- 6 (where N= 18 is the number of atoms in the molecule). Twenty six of these fundamental modes are associated with class A’ and 22 with class A”. All the absorptions are expected to be both IR and Raman active as the molecule lacks a centre of symmetry. The gauche rotamer with C,
117
symmetry would have 48 A-type fundamentals which are both IR and Raman active. Comparison of the IR and Raman spectra of the compound in the solid state shows that many absorption bands coincide in frequency. This is to be expected since the conformations in which the molecule can exist do not possess any centre of symmetry. However, the presence of an extra IR band at 442 cm-’ and an extra Raman band at 2940 cm-’ in the spectra of the compound in the solution state suggest strongly that at least two rotamers are present in solution while only one rotamer is present in the solid state. The band at 442 cm-’ was found to decrease in intensity with increasing polarity of the solvent and may thus be attributed to the non-polar rotamer (1). Hence, the polar rotamer (2) or (3) where the nitro groups are gauche to each other appears to be the stable form in the solid. This is consistent with most of the dinitro compounds studied so far [3,13]. 2-Methyl-1,2-dinitropropane differs from 2,3-dimethyl-2,3dinitrobutane only in having two hydrogen atoms in place of two methyl groups. In making spectral assignments, advantage has been taken of our earlier study on the latter compound together with papers on its bromo and chloro analogues [ 15,161 and on isobutane [ 171. Comparing the spectra of 2-methyl-1,2_dinitropropane with those of 2,3-dimethyl-2,3dinitrobutane, extra bands are found at 908, 1275,2470,2750 and 2855 cm-‘. The band at 908 cm-l with its characteristic strong intensity may be identified with the CH, rocking modes while the absorption at 1275 cm-’ can be attributed to the CH, twisting and wagging vibrations. The bands at 2470 and 2750 cm-l may arise from combination bands due to the C-H bending modes while the band at 2855 cm-’ is probably due to the symmetric CHz stretching vibrations. The bands associated with the C-C stretching and CH, rocking vibrations are expected in the spectral region 1250-710 cm-‘. Coupling between each of the C-C vibrations in the skeleton together with the coupling of the C-C stretching modes with the CH, rocking modes make the assignments rather difficult. The C-NO, stretching modes are known to give rise to strong absorption in the 1560-1340 cm-l region [ 181. This result is confirmed particularly by the Raman data. The bands at 1565 and 1354 cm-’ of the IR spectra and at 1555 and 1345 cm-’ of the Raman spectra are assigned to the respective antisymmetric and symmetric C-NO, stretchings. Diallo [ 19 ] has attributed the absorptions in the range 1060-850 cm-’ for dihalogeno-dinitromethanes to C-N stretchings. Similarly, the two relatively strong bands between 865 and 835 cm-l have been assigned to the C-N stretching. This assignment is reasonable because isobutane and the dihalogeno derivatives of this compound (in which C-N bonds are absent) do not have any strong absorptions in this region. The broad bands in the 650-610 cm-l region have been assigned to the bending modes of the nitro group. The bands in this region could also be assigned to
3030(1.5) 3000(7) 2950(l) 2925(8.5) 2890(2) 2855 (0.75)
d
3030(7) E
1525sh ( 1) 1472(14) 1460(3) 1445(4.25) 1430(6.25) 1405(20) 1378(37)
d
1460(17) 1455(0.5) 1440(0.5) 1426(7) 1407 (20)
1560br(47)
d
d
d
d
d
2750(3.75) 2695(5.25) 2470(3.75) 2405(3.25) 1565br(38)
c
c
HB
Nujol
2739(7.75) 2680(6.75) 2465(5.5) 2395(6.75) 1580(76) 1570sh(5) 1528(36) 1470(14) 1456(16) 1440(13) 1417(9.75) 1402(18) 1375(40) 1398(32) 1370(57)
e
2735(3.5) 2680(2.75) 2470(1.75) 2395(4) E
1440(17) 1418(17.5) 1395 (17.5) 1385(63)
c
2740(7.75) 2685 (6.75) 2465 (2) 2405(8.5) 1570(88) 1555(2) e
d
1470( 13) 1458(2.25) 1440(4) 1424(1.5) 1405(11) 1375(35)
d
d
2740(5) 2685 (3.5) 2465(3.5) 2410(3.5) 1565br(48)
d
d
d
2930(10) 2890 (9 )
2750(3.5) 2690(2.5) e e e e e e c e e e e
d
c
c
c
e
2990(19) 2950(22) 2930(7) 2890( 13)
c e
e
c
e
3020(4) 2995 (33.25) 2945(12.25) 2930 (2.25) 2895(9.5)
CH,CN (3.0 wt.%) 37.P
3020(4.75) 2995(55) 2945(19) 2930(8) 2890(17)
CHCl, (3.0 wt.%) 4.806b
GH, (1.9wt.%) 2.284b
CR, (1.1 wt.%) 2.641b
cc14 (3.0 wt.%) 2.23ab
IR spectrum* of 2-methyl-1,2_dinitropropane
TABLE 1
i
i
SC& ( sym )
vNOz (asym)
Combination
vCI-Is(sym 1
uCH, (asym ) & VCH,(sym)
Assignment
530(12.75) 459(10.25)
d
e
836(3) 752(7.25) 708(5.75) 636(l) 612br(2) 575br( 12) 530(3) 455(5.25)
907 (5.75)
650( 14) 633br(12.5) 586(3.75) 527(25.5) 458(7.5) 442(3)
1240(25.5) 1182(17) 1146(19.75) 1023(10) 990(2.75) 949 (8.5) 933(19) 904(35.25) 858(61) e c c
1345 (25.75) 1271(44) 1242 (25) 1181(8.25) 1147(9) 1023 (4.75) 996(2.5) 949(3.25) 933(14) 904(21.5) 858(41.25) 830( 16.5) 755(23) 708( 12) 651(U) 625br(3) 586(2) 528(13) 465(l) 445(3)
1346(49.75) 1273 (32)
528(18.75) 464(4) 449(3.5)
e E e c c
933(15) 906(25.5) 860(41.25) 830(10)
1239(31) c c c c c
1348(44.75) 1273(43)
d
e c c
445(6)
630br(12) 586( 1.5) 528( 17)
996(5.25) 950(7.25) 930(8) 904(22.5) 860(39) 830(8.25)
1148(34) e
c c
1350(13) 1273 (37)
950(3) 935(3.75) 903(5.5) 865(46.5) 835( 19.5) 760(27) 710(19.75) 640sh(1.5) 620(6.5) 580(0.5) 529(7.5) 465(4) 444(3)
1250(22.5) 1185(13) 1155(3) e e
1275 (40)
c
LX
PNC,
UC-C,PCH,
vCN
pCH, (rock)
PCH,, UC-C
JM&(sym)
CH2 (twist and wag)
“Absorption peak heights (and hence approximate relative intensities) are given in parentheses. “ezOvalue. “Masked by solvent. dAbsence of absorption.
d
d
1247 (8) c
1245(6.75) 1185(17) 1155(9) 1030(11.25) lOOO(5) 953(7.75) 937(17.75) 908(21.75) 865 (27.75) 835 (19.75) 752(22.25) 710(17.75) 636(3) 613br(8.5)
1151(l) 1029(2.75) e e e
1351(23.75) 1275(12)
1354(19) 1275(19)
120 TABLE 2 Raman spectrum” of 2-methyl-1,2-dinitropropane Solid
CHCl, (35 wt.%) 4.806b
CsHs (40 wt.%) 2.284b
CH,CN (61 wt.%) 37.5b
CCI, (36 wt.%) 2.238b
CS, (40 wt.%) 2.641b
3010(15) 2970(16.5) 2955(20.5) d
c 2970(18) 2950(25.5)
3010(13) ’ c
= e e
3002(17.5) 2975(24.5) 2955(32)
3005(12.5) 2980(18) 2955(26)
2875 (6)
2940(18) 2875(5.5)
2940(21) 2875(4)
c ’
2930(24.5) 2880(10.5)
2940(18) 2870(4)
2745(5) 1555( 17) 1455(18) 1440(18) 1420(16)
2740(3.5) 1555(17) 1455(16.5) 1440(15) 1420(15)
2750(3.5) 1560(20) 1460(13) 1442(13) ’
’ 1557(22.5) ’ ’ c
2740(8) 1555(19.5) 1455(18.5) 1445(20) 1420(19)
2740(2) 1555(15) 1460(14) 1445 (16.5) 1420( 16)
1400(23.5) 1375(46) 1345 (35) 1270(8) 1240(7.5) 1180(8) 1145(7) 1025(6.5)
1400(30) 1372(53)
’ 1375(57)
1402(33.5) 1372(61)
1345(36) 1275(8.5) 1240(10) 1182(11) 1137(8) 1027(6)
1348(36) 1272(7.5) 1245(6.5) ’ 1150(13) ’
1405(37) c 1350(41) 1280(8.5) 1245(11) 1182(12) 1150(11) e
1345(43.5) 1275(10.5) 1240(9) 1180(11.5) 1150(10) 1025(8)
1402(32) 1375(58.5) 1347(40.5) 1270(7) 1235(6) 1180(7.5) 1150(5) 1027(4)
950(14.5) 937(20)
lOOO(13) 952(18) c
995(15) 950(11.5) 935(24.5)
1000(10) 950(14) 935(20.5)
907(36)
’
905(47.5) SSO(26) 835(100)
907(43) 860(19) 835(100)
995(12) 950(14.5) 935(17)
995(11.5) 950(13) 935(21)
c
905(33) 860(17.5) 835 (80.5)
905 (38) 860(19) 835 (96.5)
’
802(g) 755(14) 710(8.5)
800(13.5)
’
c
637(11.5) 612(13) 580(23) 555(25.5) 525(29) 440(19.5) 365(40) 335(30) 265(33)
625(12.5) 610(12.5) 580(26.5) 560(27) 530(31.5) 442(20) = 335(31.5) ’
865(20) 835 (94) 802(11)
835 (94.5) 760(15) 710(7.5)
710(12) ’ ’
585(27.5) 560(27) 530(30) 440(19) 370(42) 337(27) 267(26.5)
’ d
’ c
’ 337(30) 270(27)
755(17) 710(7)
710(U)
710(10) 610(13) 580(35) 560(35) 535(35) 445(24)
c
637(14) 610(16) 580(37) 560(37) 527(40.5) ’ 365(48) 332(40) 265(35)
’
Assignment
~CH,(asym),~CH,(~ym)
vCH,(sym)
i
Combination vNO, (asym) GCHa(=ym)JCH,
=H,(sym)
i
vN0, (sym ) TCH, (twist and wag)
PCHQC-c
pCJ%hC
1 PCHZ YCN > vc-c,pCH,
617(14) 580(30) 555 (30.5) 530(37.5)
PNO,
447(22) 365(46.5) 335(30) 265(29)
> PC-C-C
“Absorption peak heights (and hence approximate relative intensities) are given in parentheses. bt~~value. ‘Masked by solvent. dAbsence of absorption.
the C-C vibrational modes of the central and non-central carbon atoms. In addition, coupling between the NO, bending modes and the skeletal deformation modes may occur.
121 TABLE 3 Polarizations, dinitropropane’
refractions,
Temp. (“C)
Solvent
7 25 45 7 25 45 25
Benzene Benzene Benzene ccl, ccl, ccl, Cyclohexane
dipole momenta
Cont. range ( l@w, )
(Ye,
520-1390 830-5120 469-1891 540- 900 400- 800 300- 870 200- 550
17.79 15.69 14.11 27.90 24.77 22.17 10.62
and molar Kerr constants
/I
0.376 0.318 0.362 -0.274 -0.265 -0.249 0.356
Y
6
-0.026
-5.48
-0.018
-40.33
at infinite
dilution
of 2-methyl-1,2-
(cm3) (ems)
1o,,p (Cm)
lo27 (mK,) (m5V-2mol-1)
508.2 471.6 442.7 459.2 422.8 397.0 408.7
15.59 15.45 15.41 14.76 14.56 14.53 14.28
Pz
Rn
30.8
31.9 32.4
- 151
-75
“Incremental changes in the relative permittivites, densities, refractive indices and Kerr constants (de, Ad, An and AB respectively) were measured for solutions having solute weight fractions q. The coefficients (Y,/?, y and Gwere derived from the relations (it,= IA~JI&II,,~~,= EAd/&a,, ynf = XAn2/Ew, and 6B,= ~AB/Iw,. %alculated on the basis that Dp= 1.05Ro.
H
NO2 (1) trans
(2) gauche
Ii (3) gauche
Fig. 1. Newman projections of 2-methyl-1,2-dinitropropane.
Comparison of solution-state spectra
Although all the bands of the solid may be attributed to the gauche rotamer, it is interesting to note that many of these bands decrease in intensity on dissolution in more polar solvents. These bands may be due to the extra trans bands which appear in solution, coinciding with the gauche bands. Such bands attributable to a mixture of gauche and trans rotamers are at 2995,2739,2680, 2395,1580,1456,1440, 1417,1402,1375, 1345, 1271,933,904 and 650 cm-l. Besides these bands, there are a few bands which decrease in intensity on dissolution in the non-polar solvents but increase in intensity with increasing
122
polarity of the solvent. These are the purely gauche bands which occur at 2945, 835,755 and 708 cm-‘. The intensities of these gauche bands however do not vary very significantly with changes in the polarity of the solvents. This implies that there is not much change in the population of gauche rotamer on changing the solvent, probably because the population of the gauche rotamer is already close to saturation. Hence from the IR and Raman spectra of the compound, we conclude that 2-methyl-1,2dinitropropane exists exclusively in the gauche conformation in the solid state, while in solution a mixture of gauche and truns rotamers is present with the truns rotamer increasing in proportion as the polarity of the solvent decreases. Dipole moment and Kerr effect measurements Results of the dipole moment measurements in benzene, carbon tetrachloride and cyclohexane at various temperatures are summarized in Table 3. Kerr effect measurements in benzene and carbon tetrachloride are also included. Table 3 shows that the dipole moments of 2-methyl-1,2dinitropropane in carbon tetrachloride and cyclohexane are very similar and hence the average of these values may be considered to be the dipole moment of the compound in the absence of solvent/solute interaction. The large dipole moment means that the polar rotamer (2) or (3) must be present in high proportion in these solvents. Table 3 also shows that the dipole moment of the compound decreases with increasing temperature in carbon tetrachloride. It follows from Boltzmann statistics that the gauche rotamer is more stable than the truns, and is greater in population in this solvent. Analysis of our dipole moment data in Ccl, according to the Lennard-Jones-Pike method [20] gives an energy difference (A& =Ep -Et) between gauche and truns of -7.172 kJ mol-l. The dipole moment of the gauche rotamer (Pi) obtained from this analysis was 15.49 x 10d3’ Cm. The dihedral angle of the gauche rotamer may be calculated from its dipole moment as follows: the resultant moment p (0) of any rotamer of a substituted ethane XCR,-CR2 X is given by (I) where pi and b are the moments of the (C&X) and (C&X) moieties of the molecule respectively; (Y~and cry2 are the supplementary angles of the central C (1 )-C (2)-X and C (2)-C (1 )-X bond angles respectively and 8 is the dihedral angle between the two C-C-X planes. In these calculations, ,ul was taken to be the value of the dipole moment of 2-nitropropane (11.74~10e3’ Cm) [ 11, p2 the moment of nitromethane (10.45 x 10e3’ Cm) [ 211; from the optimization of the geometry of 2-methyl-1,2dinitropropane, czl and (Yewere found
123
to be 70.3” and 64” respectively, using AMY Since asp= 15.49 X 10P3’ Cm, substitution of these values in eqn. (1) gives 8= 81.6”. These values of AE, and 8 correspond to a gauche rotamer population of 88% at 25°C. The very high proportion of the gauche rotamer suggests that in this compound the gauche conformation is inherently much more stable than the ~M~LS,despite the expectation that the two bulky nitro groups would tend to avoid each other to the maximum extent by adopting the tram conformation. This stability appears to be another manifestation of the “guuche effect” noted by Wolfe [ 221; it is consistent with the theory that there is a tendency for that structure to be favoured which has the maximum number of gauche interactions between the adjacent electron pairs and/or polar bands. In 2-methyl-1,2dinitropropane, each nitro group has a very high polarity of ca. 11.68 x 10w3’ cm and possesses two electron pairs on each oxygen atom and one electron pair on the nitrogen atom. However the final conformation adopted must be decided by the overall balance of attraction and repulsion between all the groups in the molecule. The variation of the heat of formation obtained from AM1 calculations with the NO,-C-C-NO, dihedral angle is given in Fig. 2. Clearly the gauche rotamers have a lower energy than the trum form. The energy difference between theguuche and truns forms obtained from the calculations was - 6.53 kJ mol-’ and the torsion angle of the gauche rotamer of lowest energy was found to be 66” when the geometry was fully optimized. A Gaussian 80 (3-21G) calculation on the gauche rotamer with full geometry optimization gave a torsion angle of 69”. From the Boltzmann distribution, the relative populations of the 2-METHYL-1.2-DINITROPROPANE
-55
1
-80
( 0
I
30
I
60
I
90
I
1
120
150
TORSIONAL
I
160
ANGLE
I
210
I
240
I
270
I
I
300
330
r 360
(DEGREES)
Fig. 2. Energy of 2-methyl-1,2-dinitropropane
as a function of the N-C-C-N
torsion angle.
124
two rotamers separated by an energy difference of -6.53 kJ mole1 would be 4% for the trans state and 96% for the gauche state. This trans/gau.che population ratio is in reasonable agreement with the experimentally derived populations. The difference of some 15” between the calculated torsion angle and the experimentally derived value is not entirely unexpected as the AMPAC and Gaussian 80 values represent the situation in vacua which is likely to be modified in solution. From Table 3, it can be seen that the dipole moment of 2-methyl-1,2dinitropropane in benzene also decreases with increasing temperature. From the experimental data, the internal energy difference between the gauche and trans rotamers was found to be - 8.672 kJ mol-‘. This corresponds to a population of about 94% gauche and 6% trurzs rotamer in solution. The dipole moment of the gauche rotamer was found to be 15.95 x 10m3’ Cm. This yields a dihedral angle of 77” from eqn. (1). In benzene, there is also an augmentation in the dipole moment of 0.89~ 10m3’ Cm relative to carbon tetrachloride at 25°C. Compared to other dinitroethanes studied so far this augmentation is rather small. However, the smallness of the augmentation can be explained because the gauche rotamer is already extensively stabilized. Nevertheless, the existence of a significant difference between the mK values in benzene and carbon tetrachloride suggests that some solvent/solute interaction is present probably in the form of n-complex formation between the “acidic” hydrogens of the solute and the solvent benzene molecules. The negative AmK value ( - 76 x 10z7 m5 Ve2 mol-’ ) further indicates that the resultant dipole moment of the solute is at a large angle to the plane of the solvating benzene molecules as shown below.
Table 4 shows the free energy and enthalpy values for the trans=guuche equilibrium for 2-methyl-1,2-dinitropropane as derived from the variation of dipole moment values with temperature. In both solvents, the difference between m” and AE [MO = A&+ A(PV) ] is positive, implying an increase in volume of the system when the trans molecules are converted to the gauche form under constant pressure. AM1 calculations indicate that although the optimized geometries of the trans and gauche rotamers have similar bond
125
TABLE 4 Thermodynamic quantities governing gauche- trans equilibrium (kJ mol- ’ ) Solvent
Temp. (“C)
K= NJN,
AGO”
ccl,
25 45 7
7.60 7.38 9.87
- 5.026 - 5.285 -5.331
I
Benzene
25 45 7
14.94 13.79 20.89
- 6.699 --7.076 6.939
I
Awb
AE
AHO-AE
-5.215
-7.172
1.957
- 6.908
- 8.672
1.764
“AGOis calculated from the relation AGO= - RTlnK. bmo is obtained from the slope of the 1nK versus l/T plot by the method of least squares and assuming a0 to be constant over the temperature range.
lengths, the C-C-N bond angle of the gauche form is about 3 ’ larger than that of the trans form. This increase in bond angle is larger than the increments found in the other bond angles of the molecule and could probably be due to steric interactions between the bulky nitro groups. Hence the increase in volume when the truns rotamer is converted into the gauche conformation could be due to this increase in bond angle. On the other hand, in benzene solution, the difference between AHo and AE is slightly less positive compared to the corresponding value in Ccl, solution. This is in agreement with our spectroscopic and molar Kerr constant results which show that although the gauche molecules are capable of attracting the solvent benzene molecule to form n complexes more effectively than the trans, the small increase in gauche population on changing the solvent from carbon tetrachloride to benzene would not much affect any physical property due to the presence of the gauche rotamer. ACKNOWLEDGEMENT
We thank Dr. David Winkler for helpful discussions.
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