- Email: [email protected]
Molecular mechanics calculations for conformational energies and torional force constants in fluoro propenes and butenes
of Molecular Structure, 161 (1987) 283--296 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Journal
MOLECULAR MECHANICS CALCULATIONS FOR CONFORMATIONAL ENERGIES AND TORSIONAL FORCE CONSTANTS IN FLUORO PROPENES AND BUTENES
PER J. STAVNEBREKK Department
of Chemistry,
and REIDAR SWLEVIK AVH,
University
of Trondheim,
N-8055
Dragvoll
(Norway)
(Received 9 March 1987)
ABSTRACT Experimental data on conformational energies of the molecules FH,C-HC=CH,, FH,C-FC=CH,, FH,C-(CH,)C=CH, and trans-FH,C-HC=C(CH,)H have been used to establish parameter values for the nonbonding atom. . . atom interaction F . . C(sp’) within the Morse potential formulation. Torsional potentials have been calculated for the four molecules mentioned above and in addition for cis- and trans-FH,C-HC=CHF, cis-FH,C-HC=CH-CH,F, CH,-FCH-HC=CH, and FH,C-CH,(FH,C),C=CH,, HC=CH, . Calculated results have been compared with experimental values. Torsional force constants for the molecules have been obtained. A comparison between fluoro,
chloro and bromo compounds is presented. INTRODUCTION
Parameter values for the nonbonding interaction F . . * C(sp*) were derived in previous work [l] . However, the parameter values for that interaction were derived from fluoro substituted 1,3-butadienes and biphenyls. This work is based on results from spectroscopic studies on four fluoro compounds. Previously established [2] parameter values for the interactions H. *. H, . . C, H * * * F, F . . . C(sp3) and F * . . F were used in order to establish more H* reliable parameter values for the potential F . . . C(sp*) suitable for propene and butene derivatives. Parameter values for the interaction C(sp’) * . . C(sp3) for the butenes were estimated in work on chloropropenes and butenes [ 31. Calculations on the chloro and bromopropenes and butenes, which are the analogues to the fluoro compounds presented here, have already been carried out [3, 41. The parameters used here are those relevant to the Morse potential formulation (Table 1) [5] . CALCULATIONS
AND RESULTS
Force constants and reference values used here are found elsewhere [l, 6, 71 . Some new reference values have been introduced recently [4] replacing some of the earlier values from ref. 1. 0022-2860/87/$03.50
o 1987 Elsevier Science Publishers B.V.
284 TABLE 1 Parameter values of non-bonded formulationa
atom . . . atom interactions
Interaction
R,(a)
%,
F . * * C(sp’) C(sp”) . . . C(sp’p
2.60 3.00
3.10 3.43
(a)
in the Morse potential(V)
E (kcal mol-‘) 1.50 0.083
aThe parameters R,, R, and E are related as follows: V(R,) = 0, V(R,) ponds to minimum of V(R); see ref. 5 for details. bValues from ref. 3.
= -E corres-
In addition, experience from recent work [3, 41 has dictated a few changes in the values being used here. Reference values and force constants are listed in Table 2. Excess charges on the atoms were computed as suggested by Sanderson [8] , but reduced by division by 2.0 for the fluoro compounds. The parameters D of the electrostatic energy terms D/R are given in Table 3. An intrinsic torsional potential for rotation around the CC for bonds V = 0.5V3(1 - cos(3@)) was used with V3 = 2.11 kcal mol-’ C(sp’) - C(sp”) and V3 = 2.65 kcal mol-’ for C(sp”) - C(sp3). Values of the new potential parameters are given in Table 1. The values for F0.e C(sp”) given here should be considered more relevant for fluoro propenes and butenes than those of ref. 1. The values in Table 1 are those for which the MM calculations reproduced the observations reasonably well. Energy minima were found by refining bond lengths, bond angles and torsion angles simultaneously. When calculating the energy of a point not corresponding to a minimum the torsion angles were held constant.
3-Fluoropropene Torsional potential with conformational
curves for H&.+CH-CH2F are shown in Fig. 1 together drawings. Our calculated energy difference is AE = E(gauche) - E(syn) = 0.5 kcal mol- l. For this molecule several spectroscopic investigations were undertaken. Two early studies using MW [ 9 ] and MW combined with far-infrared spectroscopy [lo] gave a AE value of about 0.2 + 0.1 kcal mol-‘. Two later studies using far infrared and Raman spectroscopy [ll] and far-infrared, Raman and MW spectroscopy [12] give an energy difference of about 0.8 f 0.1 kcal mol-‘. Thus, our calculated value of AE lies between these two pairs of investigations. Our calculated values of the barrier heights syn -+ gauche, gauche + syn andgauche +gauche are 1.8, 1.4 and 2.2 kcal mol-’ respectively. The corresponding experimental values [12] are 3.1, 2.2 and 2.9 kcal mol-‘. The effect of not including the electrostatic terms in the calculations are quite pronounced, as seen from Fig. 1. Calculated values for the structural parameters have been compared with experimental values [9] in Table 4.
285 TABLE 2 Reference values and force constants in fluoropropenes
and butenes
Force constants
Reference values
Bond
mdyn A-’
r” (a)
C=C ==C-C -C-C(Cl=C2-C3-C4) C-F(CH,F) =C--F C-H =C-H --C-H(C1=C2-C3-C4)
8.20 5.00 4.81 6.03 6.00 4.82 5.16 4.14
1.336 1.501 1.513 1.390 1.323 1.100 1.090 1.100
Bond angle in Cl=C2-C3
mdyn a rad-’
O” (deg.)
Cl=C2-F, C=C-H C=C-C(H C=C-C(C C=C-C(F C2-C3-F C2-C3-H H-+3-F H-C3-H
1.00 0.55 0.70 0.70 0.70 0.83 0.71 0.86 0.53
122.0 120.0 123.3 121.5 125.0 111.0 109.5 109.5 109.5
0.83 0.71 0.86 0.90 0.83
109.5
CB=Cl-F at 2) at C2) at C2)
Bond angle in Cl=C2-C3-C4 C3<4-F C2-C3-H, C3-C4-H H+3-F, H-X4-F C2-C3-C4 C2-C3-F
109.5 109.5 111.5 111.0
TABLE 3 Coulomb parameters D = 332qq*
in kcal A mol.’
(q and q* are excess charges)
H...H
C...H
H,C=CH-CH,F
0.95
0.39
-3.17
-1.32
H,C=CF-CH,Fa H,C=C(CH,)+ZH,Fb H,C=C(CH,F), ’
1.30 0.28 0.79
0.64 -0.02 0.28
-3.55 -1.90 -2.95
-1.75 0.11 -1.06
H...F
C*..F
F...F -
c...c -_
9.76 11.02
0.01 -
aSame values used for cis- and trans-FHC=CH-CH,F. bSame values used for trcns-CH,HC=CH-CH,F, H,C=CH-CH,-CH,F and H,C=CH-CHF-CH,. CSame values used for cis-FH,C-HC=CH-CH,F.
286
0
30
60
90
120
150
180
Fig. 1. H,C=CH---CH,X. X = fluorine. Torsional potential curves E (kcal mol-‘) for 3fluoropropene. The torsion angle o(C=C-C-X) is in degrees. The conformer syn (S) possessing C, symmetry has a planar arrangement of the atoms CCCX with @ = 0”. The transition form anti (A) also has a planar arrangement of the atoms CCCX with o = 180”) while the gauche (G) form has o = 120”. The dotted curve was obtained without excess charges on the atoms.
TABLE 4 Results for H,C=CH-CH,F Cl=C2-C3
Calculated values
Observeda values [ 9 ]
vn
gauche
1.506 1.390
1.502 1.388
1.48811.486 1.38211.371
Bond angles (degrees) C-C==C 124.8 C=C-H 119.7 C-C-F 112.1 C-C-H 109.1 F-C-H 108.9
124.0 119.9 109.9 109.1 109.1
124.6/121.6 119.0 111.7/110.9 108.1/111.3 107.3/107.1
Torsion angles (degrees) @J (C==C-C-F) 0.0
116.3
127 f 1
Bond lengths (A) C-C C-F
Torsional force constants in mdyn A rad” F = a’Elao’ 0.055 0.056 aUncertainty in bond lengths ‘v 0.010 A, in angles 0.5-1.5”.
287
cis-1,3-Difluoropropene Calculated torsional potential curves for cis-FHC=CH-CH,F,
together
with conformational drawings are shown in Fig. 2. The calculations predict the existence of two stable conformers, the gauche form being 0.2 kcal more stable than the syn. For the chloro and bromo compounds, mol-’ cis-1,3dichloropropene and cis-1,3dibromopropene, only the gauche form is found to be stable. (See refs. 3 and 4 and references therein.) The calculated syn + gauche barrier height is 1.1 kcal mol-‘. The effect of the electrostatic terms on the calculations should be noticed for this compound (Fig. 2). Calculated values of the torsional force constants in mdyn A rad-’ are: 0.038 (syn) and 0.052 (gauche). For the torsion angles the values 0” (syn) and 115.7” (gauche) were obtained. trans-1,3-Difluoropropene
As expected
the torsional potential curve for this molecule is similar to in Fig. 1. The calculated energy difference AE = E(gauche) E(syn) was 0.5 kcal mol-’ . Neglecting the electrostatic terms in the energy calculations did not change the results significantly for this compound. Calculated values of the torsional force constants in mdyn a radW2 were 0.055 (syn) and 0.056 (gauche). The values 0” (syn) and 116.4” (gauche) were obtained for the torsion angles. that
0
shown
30
60
90
120
150
160
Fig. 2. cis-XHC=CH-CH,-X. X = fluorine. Torsional potential curves E (kcal mol-I) for cis-1,3difluoropropene. The dotted curve was obtained without excess charges on the atoms. The torsion angle @(C=C-C--X) is in degrees. The two forms syn (S) and anti (A) have the atoms CCCX in planar arrangements with @(syn) = 0” and @(anti) = 180”. Both A and S possess C, symmetry, while G has C, symmetry.
288
2,3-Difluoropropene Torsional potential curves together with conformational drawings for H2C=CF-CHzF are shown in Fig. 3. Our calculated energy difference is E(guuche) - E(syn) = 0.8 kcal mol-‘, while the value from MW and infrared spectroscopy is 0.4 + 0.2 kcal mol-’ [13]. Thus, our value is slightly outside the stated error limits of the experimental values. The results for this molecule are very sensitive to the values of the electrostatic terms. A reduction of the charge values by about 50% would lead to complete agreement between calculated and observed results. Calculated values of the torsional force constants in mdyn a rad.-* are 0.059 (syn) and 0.062 (gauche). The values obtained for the torsion angles were 0” (syn) and 113.6” (gauche). trans-1 -Fluoro-2-butene The torsional potential for truns-CH,-CH=CH-CH*F and conformational drawings are shown in Fig. 4. From our calculations we obtained an energy difference AE = (gauche) - E(syn) equal to 0.3 kcal mol-‘. The corresponding experimental value, obtained from far-infrared, Raman and MW spectroscopy [12], is 0.9 kcal mol-‘. Thus, our calculations agree that the syn conformer is the more stable, but our AE value is rather low compared with the experimental result. The potential curve obtained when the electrostatic terms were omitted, differed little from that shown in Fig. 4, but the AE value was reduced to 0.1 kcal mol-‘. The calculated values of the torsional force constants in mdyn A rad-* are
Torsional potential curves E (kcal mole’) for Fig. 3. H,C=CX-CH,X. X = fluorine. 2,3difluoropropene. The dotted curve was obtained without excess charges on the atoms. The torsion angle @(EC-C-X) is in degrees. The conformer syn (S) and the transition form anti (A) have planar arrangements of the atoms CCCX with @(syn) = 0” and @(anti) = 180”. Both S and A possess C, symmetry.
289
Fig. 4. truns-CH,-HC=CH-CH,X. X = fluorine, M = CH, group. Torsional potential curve E (kcal mol-‘) for truns-1-fluoro-2-butene. The torsion angle @(C=C-C-X) is in degrees. The conformer syn (S) and the transition form anti (A) have a planar arrangement of the atoms C-C=C-C-X with ~(syn) = 0” and @(anti) = 180”. Both S and A possess C, symmetry.
0.052 (syn) and 0.056 (gauche) for the rotation of the FH,C-group, and 0.060 for the CH,-group. For the torsion angle r$1(C=C-C,-F) the values 0” (syn) and 116.3” (gauche) were obtained, and $4(C-C-C4-H) = 0” 3-Fluoro-2-methyl-I-propene The calculated torsional potential curve for H,C==C(CH3)SH2F and conformational drawings are shown in Fig. 5. The potential curve obtained when the electrostatic energy terms were not included in the calculations is quite similar to that in Fig. 5. Our calculations predict that syn and gauche are stable conformations. The values of the gauche torsion angles are o3 (C=CC-F) = 117” and @4(C=C-C4-H) = 0.1”. An experimental study of the far-infrared spectrum of gaseous 3-fluora-methyl propene has been carried out [ 121. The data clearly indicate the existence of both syn and gauche conformers. However, determination of the more stable conformer was not conclusive. The experimental data are consistent with a barrier height (kcal mol-l) of 3.1 for the transition syn to gauche, and 2.1 for the transition gauche to gauche, while the calculated (Fig. 5) values are 2.0 and 2.4 kcal mol-‘. Furthermore, the experimental data are consistent with the syn conformer lying 0.4 kcal mol-’ lower in energy than the gauche conformer. From Fig. 5 it is seen that syn has an energy of 0.6 kcal mol-’ in excess of gauche. This discrepancy cannot be removed by changing the F.. . C(sp”) interaction potential, without simultaneously changing the calculated results for FH,C-HC=CH2, FH,C--FC= CH, and Pans-FH,C-HC=C(CH,)H in an unwanted way. For the torsional force constants the values set out in Table 5 were obtained (mdyn _i%rad-*).
290
0
30
60
90
120
150
180
Fig. 5. H,C=C(CH,)-CH,-X. X = fluorine, M = CH, group. Torsional potential curve E (kcal mol.‘) for 3-fluoro-2-methyl-l-propene. The torsion angles @$ (Cl=C2-C!3-X) and o4 (Cl=C2--C4-H’) are in degrees. The conformer syn (S) has @3 = 0” and e4 = O”, corresponding to planar syn arrangements of the atoms C=C-C3-X and C=C-C4-H simultaneously. The conformer gauche (G) has a4 = 0.1” and @J = +117”. The energy values of the curve correspond to ~1~= 0” and Q, = @)3 as variable. TABLE
5
FH,C-group CH, -group
sw
gauche
0.050 0.063
0.069 0.063
3-Fluoro-2-fluoromethyl-l-propene Conformational drawings and nomenclature for H2C=C(CH2F)2 are shown in Fig. 6A. Torsional potential curves are shown in Fig. 6B. Our calculations predict four stable conformers for this molecule. The high-energy form is SS while the most stable form is GG. However, the two other conformers GG” and GS are only 0.2 and 0.7 kcal mol-’ higher in energy than GG respectively. The SS form has energy 1.4 kcal mol-’ above that of GG. The values in Table 6 were obtained for the torsion angles (o) and torsional force constants (F). The torsional potential was little changed when the electrostatic terms were omitted in the calculations. cis-1,4-Difluoro-2-butene Conformational drawings are shown in Fig. 7A and torsional potential curves in Fig. 7B. According to our calculations the low-energy conformer is GS. Only GS and GG are stable conformers. The GG conformer is 0.6 kcal mol-’ higher in energy than GS. The unbroken curve in Fig. 7B has two minima. However, the point at $ = 40” corresponds to a saddle point on the potential surface. The GG”
291
GG
GG"
Fig. 6. (A) H,C=C(CH,X),. X = fluorine. Possible conformations of 3-fluoro-2-fluoromethyl-I-propene. In SS the fragments Cl=C2-C3-X and Cl=C2-C4-X are both planar (syn), corresponding to torsion angles @,(C=C-C3-X) = 0” and @,(C=C-C4-X) = 0”. The form GS has oJ = 120” and o4 = 0” , and GG has 0, = 120” = 04, while GG” has o3 = 120” and o4 = -120”. Thus, the symmetries are C,.,, C,, C, and C, for the forms SS, GS, GG and GG” respectively. (B) H,C%(CH,X),. X = fluorine. Torsional potential curves E (kcal mol.‘) for 3-fluoro-2-fluoromethyl-1-propene. The conformers and their names (SS, GS, GG and GG”) are explained in (A). The transition form AS has @3 = 180” The unbroken curve (-) was obtained and o4 = 0” , and thus possesses C’s symmetry. with the restriction that os = @ = 04, and the borken (- - - -) curve with o3 = @ and @, = --@, while the dotted (. . . .) curve had o3 = $J and o4 = 0”. TABLE
6
o (degrees) @3 (C=C2-C3-F) o4 (C==C2-C4-F) F (mdyn
ss
GG
0.0 0.0
118.6 118.6
115.0 -115.0
0.2 118.0
0.070 0.070 --0.009
0.071 0.070 4.022
0.051 0.069 -0.075
GG”
GS
A rad-’
a’E/ao: a’E/ao: a’E/a@,ao,
0.051 0.051 -0.007
form did not correspond to an energy minimum for this molecule according to our calculations. The point on the broken curve at 80” is not a minimum on the potential surface. The values obtained for the torsion angles (@) and torsional force constants (F) are set out in Table 7. When the electrostatic terms were neglected in the calculations, the AE value increased to 1.8 kcal mol-‘. The minimum for GS was moved to o, = 49.8” and & = 12.4”. The potential was quite flat in this region. 3-Fluoro-1
-butene
Results for H,C=CH-HCF-CH3 are shown in Fig. 8. The most stable conformer according to our calculations, is GH with F eclipsing the double bond. However, GX with an H atom eclipsing the double bond is only 0.1 kcal mol-’ higher in energy. The third conformer (SM) is about 1.0 kcal
E
SS
G-5
.. xx
52
x
x
..
x
._ --v x
\
,.
5-t 4
x‘
?=t GG
Fig. 7. (A) cis-XH,C-HC=CH-CH,X. X = fluorine. Possible conformations of &s-1,4difluoro-2-butene. In the form SS the fragments X-Cl-C2==C3 and X-C4-C3=C2 are both planar (syn), corresponding to torsion angles 0,(X-Cl-C=C) = 0” and 0,(X-C4C=C) = 0”. The form GS has @, = 90” and 9, = 0”, and for GG $+ = o4 * 90”) while for GG” o1 * 90” and o4 = 90”. The symmetries are C,,, C,, C, and C, for the conformers SS, GS, GG and GG” respectively. (B) cis-XH,C-HC=CH-CH,X. X = fluorine. Torsional potential curves E (kcal mol-I) for cis-1,4-difluoro-2-butene. The conformers and their names (SS, GS, GG and GG”) are explained in (A). The transition form AS to C, symmetry. The unbroken curve (-) has 0, = 180” and o4 = 0” corresponding was obtained with 0, = o4 = o, and the broken curve (- ---) with o, = o and o4 = -0, while the dotted (. . . .) curve @, = 0 and o4 = 0”. TABLE
7 GS
@ (degrees) @, (F--CC2=C3) o4 (C2=C3-C4-F)
86.2 -13.1
GG
89.6 89.6
F (mdyn A rad”)
a’E/a@; azEja@: a2E/a@, a@,
0.051 0.047 0.036
0.029 0.029 0.023
mol-’ less stable than GH. The barrier may be taken from the potential curve terms were neglected, GH and SM were than GX, respectively. Calculated values force constants F are shown in Table 8.
heights separating the conformers in Fig. 8. When the electrostatic 0.1 and 0.8 kcal mol-’ less stable of the torsion angles and torsional
4-Fluoro-1 -butene Conformational drawings for HzC=CH-CHz-CH2F are shown in Fig. 9A. There are five conformers corresponding to well-defined minima of the potential curves in Fig. 9B. The calculated conformational energies (kcal mol-‘) relative to the energy of GG became: 0.16, 0.67, 1.66 and 2.13 for SG, GG*, GA and SA respectively. According to these calculated energies
293
0
30
60
90
120
150
180
Fig. 8. H,C=CH-XCH-CH, . M = CH, and X = fluorine. Torsional potential curves E (kcal mol-‘) for 3-fluoro-l-butene. The torsion angles o,(Cl=C2+3-X) and o4 (C2--C3C4-H) are in degrees. The conformers have the following values of o3 : 2.8 60” for these (SM), -120.8 (GX) and +116.5 (GH). The values of o4 are approximately conformers. Thus, in SM the CH, group eclipses the C=C bond; in GX and GH the Hatom and the X-atom eclipse the C=C bond respectively. The fragment C-XCH-CH, has a nearly staggered arrangement in each of these conformations. The unbroken curve was calculated with the restriction that o3 = +o and @4 = 60”, while the dotted curve =60”. corresponds to oa =-and@, TABLE
8 GX
GH
SM
-120.8 59.8
+116.5 60.2
+2.8 59.4
0.060 0.098 4.002
0.057 0.098 -0.002
0.068 0.154 +0.043
o (degrees)
@p(C=c--c---F) ~4 (C-C-C-H)
F (mdyn
A rad-‘)
a’E/a@: a’E/a@: aWa@,
a@,
the composition at 23” is: 46% of GG, 35% SG, 15% GG*, 3% of GA and 1% of SA. The values obtained for the torsion angles (4) and torsional force constants (F) are shown in Table 9. CONCLUDING REMARKS A summary of the conformers and the conformational energy differences within the fluoro compounds as well as a comparison with the corresponding chloro [3] and bromo [4] compounds are shown in Table 10. The chloro and bromo compounds possess the same conformers and the same order of stability. In contrast to this the fluoro compounds often have a reversed order of conformational stability. For the compounds cis-FHC=CH-CHzF and cis-FHzC-HC=CH-CH2F a greater number of conformations is stable than within the corresponding chloro and bromo compounds.
GA
GG
GG'
Fig. 9. (A) H,C=CH-CH,-CH,X. Possible conformations of H,C=CH-CH,-CH,X. SA and SG have the C atoms in a planar syn (S) arrangement. For GG, GA and GG* the C atoms have a gauche (G) arrangement. The second letter in a conformational name indicates whether X has an anti (A) or gauche (G) arrangement to the C=C bond. The torsion angles are o3 (C=C-C-C) and @a (C-C-C-X). Thus the values of the torsion angles (o:/o”,) are: SA(0/180), SG (O/60), GG (120/+60), GA (120/180) and GG* (120/-60). Only SA possesses C, symmetry. (B) H,C=CH-CH,--CH,X. X = fluorine. Torsional potential curves E (kcal mol-‘) for 4-fluoro-1-butene. The conformers and their names are explained in (A). The unbroken curve (-) was calculated with the restriction that o = o4 (C=C-C--C) and o, (C-C-C-X) = 120”. The broken curve (----) corresponds to the restriction b4 = -@ and o, = 120”, while for the dotted curve (.. . .) @4 = I#Jand o3 = 0”. TABLE
9 GG
GG*
GA
SG
SA
110.8 60.2
116.3 -60.5
117.1 181.0
0.8 62.0
0.0 180.0
0.055
0.058
0.061
0.093
0.073
0.236 0.081
0.150 0.043
o (degrees)
@, (C=c-c--o ~4 (C--C-‘-F)
F (mdyn .A rade2)
a'E/a& a’E/a& aZE/a@,a0,
0.126 -0.003
0.117 -0.007
0.093 -0.001
Values of the rotational barrier heights are found typically between 1 and 2 kcal mol-‘. However, for the transition SS -+ GS -+ GG in the molecule H2C=C(CH2F)z a barrier height less than 1 kcal mol-’ was obtained, while for the transition between enantiomeric GG forms in the same molecule a barrier as high as ‘7 kcal mol-’ was calculated. None of the molecules studied here possess a double minimum in the potential at syn or anti. If the atoms C=C--C--F or C=C-C-C have a planar anti arrangement, this is a transition form indicated by A or AS in the figures. The fluoro compounds show less variation in the structural parameter values than the analogous chloro and bromo compounds. However, the following typical differences between syn (S) and gauche (G) conformers
295 TABLE Summary
10 of conformers
and calculated
conformationalenergy
X = halogen
AEa
H,C=CH-CH,X cis-XHC=CH-CH,X tmns-XHC=CH-CH,X H,C=CX--CH,X tram-XH,C-HC=CH-CH, H,C=C(CH,)*H,X
G-S G-S G-S G-S G-S G-S GG”--GG GS-GG SS-GG GG”GG GS-GG SS-GG GH-GX SM-GX GG-GA SA-GA GG*-GA SG--GA
H,C=C(CH,X),
cis-XH,C-HC=CH-CH,X
H,C=CH-XCH-CH, H,C=CH-CH,CH,X
F +0.5 -0.2 +0.5 +0.8 +0.3 -0.6 +0.2 +0.7 +1.4 _c -0.6 +2.6 -0.1 +1.1 -1.7 +0.5 -1.0 -1.5
differences
AE in kcal mol-’
Br [41
Cl 131 -0.7
-0.8
_b
_b
4.4 +0.3 4.6 -1.4 +1.4 +2.0 +3.6 -0.4
4.3 +0.8 -0.7 -1.2 +1.6 +1.6 +3.a 4.1 _d
_e
4
+0.6 +o.g +0.4 +0.4 +1.1 +0.8
+O.B +2.2 +0.2 +0.6 +0.9 +0.9
=Energy difference AE between two conformers as indicated here; G (gauche) (syn). bOnly G is stable. ‘GG” not stable. dGS not stable. eSS not stable.
and S
exist (S-C): Ar(C-C) = 0.005 a Ar(C&-F) = 0.002 8, A(CCC) = 1” and A(C&F) = 2”. For gauche conformers the torsional force constants have values in the range 0.05-0.07 and the syn conformers have values in the range 0.040.06 mdyn a rad-’ . Neglecting the atomic excess charges in the calculations had a pronounced influence on some of the potential curves as shown in Figs. 1-3. ACKNOWLEDGEMENT
We are grateful to Norges Almenvitenskapelige support.
ForskningsrPd
for financial
REFERENCES 1 2 3 4 5 6 7 8
R. R. P. P. R. T. T. R.
St.&evik and 8. Thingstad, J. Mol. Struct., 106 (1984) 333. St$levik, J. Mol. Struct., 109 (1984) 397. J. Stavnebrekk and R. Snjlevik, J. Mol. Struct., 159 (1987) 153. J. Stavnebrekk and R. StQlevik, J. Mol. Struct., 160 (1987) 143. J. Abraham and R. StQlevik, Chem. Phys. Lett., 58 (1978) 622. Rydland, Thesis, University of Trondheim, AVH, Trondheim, 1981. Rydland and R. St&evik, J. Mol. Struct. (Theochem), 105 (1983) 15. T. Sanderson, Chemical Bonds and Bond Energy, Academic Press, New York,
19 76.
296 9 10 11 12 13
E. P. J. T. A.
Hirota, J. Chem. Phys., 42 (1965) 2071. Meakin, D. 0. Harris and E. Hirota, J. Chem. Phys., 51 (1969) 3775. R. Durig, M. Zhen and T. S. Little, J. Chem. Phys., 81 (1984) 4259. S. Little and J. R. Durig, J. Mol. Struct., 114 (1984) 385. D. English, L. H. Sharpen, K. W. Ewool, M. L. Strauss and D. 0.
Spectrosc.,
60 (1976)
210.
Harris, J. Mol.
Journal
MOLECULAR MECHANICS CALCULATIONS FOR CONFORMATIONAL ENERGIES AND TORSIONAL FORCE CONSTANTS IN FLUORO PROPENES AND BUTENES
PER J. STAVNEBREKK Department
of Chemistry,
and REIDAR SWLEVIK AVH,
University
of Trondheim,
N-8055
Dragvoll
(Norway)
(Received 9 March 1987)
ABSTRACT Experimental data on conformational energies of the molecules FH,C-HC=CH,, FH,C-FC=CH,, FH,C-(CH,)C=CH, and trans-FH,C-HC=C(CH,)H have been used to establish parameter values for the nonbonding atom. . . atom interaction F . . C(sp’) within the Morse potential formulation. Torsional potentials have been calculated for the four molecules mentioned above and in addition for cis- and trans-FH,C-HC=CHF, cis-FH,C-HC=CH-CH,F, CH,-FCH-HC=CH, and FH,C-CH,(FH,C),C=CH,, HC=CH, . Calculated results have been compared with experimental values. Torsional force constants for the molecules have been obtained. A comparison between fluoro,
chloro and bromo compounds is presented. INTRODUCTION
Parameter values for the nonbonding interaction F . . * C(sp*) were derived in previous work [l] . However, the parameter values for that interaction were derived from fluoro substituted 1,3-butadienes and biphenyls. This work is based on results from spectroscopic studies on four fluoro compounds. Previously established [2] parameter values for the interactions H. *. H, . . C, H * * * F, F . . . C(sp3) and F * . . F were used in order to establish more H* reliable parameter values for the potential F . . . C(sp*) suitable for propene and butene derivatives. Parameter values for the interaction C(sp’) * . . C(sp3) for the butenes were estimated in work on chloropropenes and butenes [ 31. Calculations on the chloro and bromopropenes and butenes, which are the analogues to the fluoro compounds presented here, have already been carried out [3, 41. The parameters used here are those relevant to the Morse potential formulation (Table 1) [5] . CALCULATIONS
AND RESULTS
Force constants and reference values used here are found elsewhere [l, 6, 71 . Some new reference values have been introduced recently [4] replacing some of the earlier values from ref. 1. 0022-2860/87/$03.50
o 1987 Elsevier Science Publishers B.V.
284 TABLE 1 Parameter values of non-bonded formulationa
atom . . . atom interactions
Interaction
R,(a)
%,
F . * * C(sp’) C(sp”) . . . C(sp’p
2.60 3.00
3.10 3.43
(a)
in the Morse potential(V)
E (kcal mol-‘) 1.50 0.083
aThe parameters R,, R, and E are related as follows: V(R,) = 0, V(R,) ponds to minimum of V(R); see ref. 5 for details. bValues from ref. 3.
= -E corres-
In addition, experience from recent work [3, 41 has dictated a few changes in the values being used here. Reference values and force constants are listed in Table 2. Excess charges on the atoms were computed as suggested by Sanderson [8] , but reduced by division by 2.0 for the fluoro compounds. The parameters D of the electrostatic energy terms D/R are given in Table 3. An intrinsic torsional potential for rotation around the CC for bonds V = 0.5V3(1 - cos(3@)) was used with V3 = 2.11 kcal mol-’ C(sp’) - C(sp”) and V3 = 2.65 kcal mol-’ for C(sp”) - C(sp3). Values of the new potential parameters are given in Table 1. The values for F0.e C(sp”) given here should be considered more relevant for fluoro propenes and butenes than those of ref. 1. The values in Table 1 are those for which the MM calculations reproduced the observations reasonably well. Energy minima were found by refining bond lengths, bond angles and torsion angles simultaneously. When calculating the energy of a point not corresponding to a minimum the torsion angles were held constant.
3-Fluoropropene Torsional potential with conformational
curves for H&.+CH-CH2F are shown in Fig. 1 together drawings. Our calculated energy difference is AE = E(gauche) - E(syn) = 0.5 kcal mol- l. For this molecule several spectroscopic investigations were undertaken. Two early studies using MW [ 9 ] and MW combined with far-infrared spectroscopy [lo] gave a AE value of about 0.2 + 0.1 kcal mol-‘. Two later studies using far infrared and Raman spectroscopy [ll] and far-infrared, Raman and MW spectroscopy [12] give an energy difference of about 0.8 f 0.1 kcal mol-‘. Thus, our calculated value of AE lies between these two pairs of investigations. Our calculated values of the barrier heights syn -+ gauche, gauche + syn andgauche +gauche are 1.8, 1.4 and 2.2 kcal mol-’ respectively. The corresponding experimental values [12] are 3.1, 2.2 and 2.9 kcal mol-‘. The effect of not including the electrostatic terms in the calculations are quite pronounced, as seen from Fig. 1. Calculated values for the structural parameters have been compared with experimental values [9] in Table 4.
285 TABLE 2 Reference values and force constants in fluoropropenes
and butenes
Force constants
Reference values
Bond
mdyn A-’
r” (a)
C=C ==C-C -C-C(Cl=C2-C3-C4) C-F(CH,F) =C--F C-H =C-H --C-H(C1=C2-C3-C4)
8.20 5.00 4.81 6.03 6.00 4.82 5.16 4.14
1.336 1.501 1.513 1.390 1.323 1.100 1.090 1.100
Bond angle in Cl=C2-C3
mdyn a rad-’
O” (deg.)
Cl=C2-F, C=C-H C=C-C(H C=C-C(C C=C-C(F C2-C3-F C2-C3-H H-+3-F H-C3-H
1.00 0.55 0.70 0.70 0.70 0.83 0.71 0.86 0.53
122.0 120.0 123.3 121.5 125.0 111.0 109.5 109.5 109.5
0.83 0.71 0.86 0.90 0.83
109.5
CB=Cl-F at 2) at C2) at C2)
Bond angle in Cl=C2-C3-C4 C3<4-F C2-C3-H, C3-C4-H H+3-F, H-X4-F C2-C3-C4 C2-C3-F
109.5 109.5 111.5 111.0
TABLE 3 Coulomb parameters D = 332qq*
in kcal A mol.’
(q and q* are excess charges)
H...H
C...H
H,C=CH-CH,F
0.95
0.39
-3.17
-1.32
H,C=CF-CH,Fa H,C=C(CH,)+ZH,Fb H,C=C(CH,F), ’
1.30 0.28 0.79
0.64 -0.02 0.28
-3.55 -1.90 -2.95
-1.75 0.11 -1.06
H...F
C*..F
F...F -
c...c -_
9.76 11.02
0.01 -
aSame values used for cis- and trans-FHC=CH-CH,F. bSame values used for trcns-CH,HC=CH-CH,F, H,C=CH-CH,-CH,F and H,C=CH-CHF-CH,. CSame values used for cis-FH,C-HC=CH-CH,F.
286
0
30
60
90
120
150
180
Fig. 1. H,C=CH---CH,X. X = fluorine. Torsional potential curves E (kcal mol-‘) for 3fluoropropene. The torsion angle o(C=C-C-X) is in degrees. The conformer syn (S) possessing C, symmetry has a planar arrangement of the atoms CCCX with @ = 0”. The transition form anti (A) also has a planar arrangement of the atoms CCCX with o = 180”) while the gauche (G) form has o = 120”. The dotted curve was obtained without excess charges on the atoms.
TABLE 4 Results for H,C=CH-CH,F Cl=C2-C3
Calculated values
Observeda values [ 9 ]
vn
gauche
1.506 1.390
1.502 1.388
1.48811.486 1.38211.371
Bond angles (degrees) C-C==C 124.8 C=C-H 119.7 C-C-F 112.1 C-C-H 109.1 F-C-H 108.9
124.0 119.9 109.9 109.1 109.1
124.6/121.6 119.0 111.7/110.9 108.1/111.3 107.3/107.1
Torsion angles (degrees) @J (C==C-C-F) 0.0
116.3
127 f 1
Bond lengths (A) C-C C-F
Torsional force constants in mdyn A rad” F = a’Elao’ 0.055 0.056 aUncertainty in bond lengths ‘v 0.010 A, in angles 0.5-1.5”.
287
cis-1,3-Difluoropropene Calculated torsional potential curves for cis-FHC=CH-CH,F,
together
with conformational drawings are shown in Fig. 2. The calculations predict the existence of two stable conformers, the gauche form being 0.2 kcal more stable than the syn. For the chloro and bromo compounds, mol-’ cis-1,3dichloropropene and cis-1,3dibromopropene, only the gauche form is found to be stable. (See refs. 3 and 4 and references therein.) The calculated syn + gauche barrier height is 1.1 kcal mol-‘. The effect of the electrostatic terms on the calculations should be noticed for this compound (Fig. 2). Calculated values of the torsional force constants in mdyn A rad-’ are: 0.038 (syn) and 0.052 (gauche). For the torsion angles the values 0” (syn) and 115.7” (gauche) were obtained. trans-1,3-Difluoropropene
As expected
the torsional potential curve for this molecule is similar to in Fig. 1. The calculated energy difference AE = E(gauche) E(syn) was 0.5 kcal mol-’ . Neglecting the electrostatic terms in the energy calculations did not change the results significantly for this compound. Calculated values of the torsional force constants in mdyn a radW2 were 0.055 (syn) and 0.056 (gauche). The values 0” (syn) and 116.4” (gauche) were obtained for the torsion angles. that
0
shown
30
60
90
120
150
160
Fig. 2. cis-XHC=CH-CH,-X. X = fluorine. Torsional potential curves E (kcal mol-I) for cis-1,3difluoropropene. The dotted curve was obtained without excess charges on the atoms. The torsion angle @(C=C-C--X) is in degrees. The two forms syn (S) and anti (A) have the atoms CCCX in planar arrangements with @(syn) = 0” and @(anti) = 180”. Both A and S possess C, symmetry, while G has C, symmetry.
288
2,3-Difluoropropene Torsional potential curves together with conformational drawings for H2C=CF-CHzF are shown in Fig. 3. Our calculated energy difference is E(guuche) - E(syn) = 0.8 kcal mol-‘, while the value from MW and infrared spectroscopy is 0.4 + 0.2 kcal mol-’ [13]. Thus, our value is slightly outside the stated error limits of the experimental values. The results for this molecule are very sensitive to the values of the electrostatic terms. A reduction of the charge values by about 50% would lead to complete agreement between calculated and observed results. Calculated values of the torsional force constants in mdyn a rad.-* are 0.059 (syn) and 0.062 (gauche). The values obtained for the torsion angles were 0” (syn) and 113.6” (gauche). trans-1 -Fluoro-2-butene The torsional potential for truns-CH,-CH=CH-CH*F and conformational drawings are shown in Fig. 4. From our calculations we obtained an energy difference AE = (gauche) - E(syn) equal to 0.3 kcal mol-‘. The corresponding experimental value, obtained from far-infrared, Raman and MW spectroscopy [12], is 0.9 kcal mol-‘. Thus, our calculations agree that the syn conformer is the more stable, but our AE value is rather low compared with the experimental result. The potential curve obtained when the electrostatic terms were omitted, differed little from that shown in Fig. 4, but the AE value was reduced to 0.1 kcal mol-‘. The calculated values of the torsional force constants in mdyn A rad-* are
Torsional potential curves E (kcal mole’) for Fig. 3. H,C=CX-CH,X. X = fluorine. 2,3difluoropropene. The dotted curve was obtained without excess charges on the atoms. The torsion angle @(EC-C-X) is in degrees. The conformer syn (S) and the transition form anti (A) have planar arrangements of the atoms CCCX with @(syn) = 0” and @(anti) = 180”. Both S and A possess C, symmetry.
289
Fig. 4. truns-CH,-HC=CH-CH,X. X = fluorine, M = CH, group. Torsional potential curve E (kcal mol-‘) for truns-1-fluoro-2-butene. The torsion angle @(C=C-C-X) is in degrees. The conformer syn (S) and the transition form anti (A) have a planar arrangement of the atoms C-C=C-C-X with ~(syn) = 0” and @(anti) = 180”. Both S and A possess C, symmetry.
0.052 (syn) and 0.056 (gauche) for the rotation of the FH,C-group, and 0.060 for the CH,-group. For the torsion angle r$1(C=C-C,-F) the values 0” (syn) and 116.3” (gauche) were obtained, and $4(C-C-C4-H) = 0” 3-Fluoro-2-methyl-I-propene The calculated torsional potential curve for H,C==C(CH3)SH2F and conformational drawings are shown in Fig. 5. The potential curve obtained when the electrostatic energy terms were not included in the calculations is quite similar to that in Fig. 5. Our calculations predict that syn and gauche are stable conformations. The values of the gauche torsion angles are o3 (C=CC-F) = 117” and @4(C=C-C4-H) = 0.1”. An experimental study of the far-infrared spectrum of gaseous 3-fluora-methyl propene has been carried out [ 121. The data clearly indicate the existence of both syn and gauche conformers. However, determination of the more stable conformer was not conclusive. The experimental data are consistent with a barrier height (kcal mol-l) of 3.1 for the transition syn to gauche, and 2.1 for the transition gauche to gauche, while the calculated (Fig. 5) values are 2.0 and 2.4 kcal mol-‘. Furthermore, the experimental data are consistent with the syn conformer lying 0.4 kcal mol-’ lower in energy than the gauche conformer. From Fig. 5 it is seen that syn has an energy of 0.6 kcal mol-’ in excess of gauche. This discrepancy cannot be removed by changing the F.. . C(sp”) interaction potential, without simultaneously changing the calculated results for FH,C-HC=CH2, FH,C--FC= CH, and Pans-FH,C-HC=C(CH,)H in an unwanted way. For the torsional force constants the values set out in Table 5 were obtained (mdyn _i%rad-*).
290
0
30
60
90
120
150
180
Fig. 5. H,C=C(CH,)-CH,-X. X = fluorine, M = CH, group. Torsional potential curve E (kcal mol.‘) for 3-fluoro-2-methyl-l-propene. The torsion angles @$ (Cl=C2-C!3-X) and o4 (Cl=C2--C4-H’) are in degrees. The conformer syn (S) has @3 = 0” and e4 = O”, corresponding to planar syn arrangements of the atoms C=C-C3-X and C=C-C4-H simultaneously. The conformer gauche (G) has a4 = 0.1” and @J = +117”. The energy values of the curve correspond to ~1~= 0” and Q, = @)3 as variable. TABLE
5
FH,C-group CH, -group
sw
gauche
0.050 0.063
0.069 0.063
3-Fluoro-2-fluoromethyl-l-propene Conformational drawings and nomenclature for H2C=C(CH2F)2 are shown in Fig. 6A. Torsional potential curves are shown in Fig. 6B. Our calculations predict four stable conformers for this molecule. The high-energy form is SS while the most stable form is GG. However, the two other conformers GG” and GS are only 0.2 and 0.7 kcal mol-’ higher in energy than GG respectively. The SS form has energy 1.4 kcal mol-’ above that of GG. The values in Table 6 were obtained for the torsion angles (o) and torsional force constants (F). The torsional potential was little changed when the electrostatic terms were omitted in the calculations. cis-1,4-Difluoro-2-butene Conformational drawings are shown in Fig. 7A and torsional potential curves in Fig. 7B. According to our calculations the low-energy conformer is GS. Only GS and GG are stable conformers. The GG conformer is 0.6 kcal mol-’ higher in energy than GS. The unbroken curve in Fig. 7B has two minima. However, the point at $ = 40” corresponds to a saddle point on the potential surface. The GG”
291
GG
GG"
Fig. 6. (A) H,C=C(CH,X),. X = fluorine. Possible conformations of 3-fluoro-2-fluoromethyl-I-propene. In SS the fragments Cl=C2-C3-X and Cl=C2-C4-X are both planar (syn), corresponding to torsion angles @,(C=C-C3-X) = 0” and @,(C=C-C4-X) = 0”. The form GS has oJ = 120” and o4 = 0” , and GG has 0, = 120” = 04, while GG” has o3 = 120” and o4 = -120”. Thus, the symmetries are C,.,, C,, C, and C, for the forms SS, GS, GG and GG” respectively. (B) H,C%(CH,X),. X = fluorine. Torsional potential curves E (kcal mol.‘) for 3-fluoro-2-fluoromethyl-1-propene. The conformers and their names (SS, GS, GG and GG”) are explained in (A). The transition form AS has @3 = 180” The unbroken curve (-) was obtained and o4 = 0” , and thus possesses C’s symmetry. with the restriction that os = @ = 04, and the borken (- - - -) curve with o3 = @ and @, = --@, while the dotted (. . . .) curve had o3 = $J and o4 = 0”. TABLE
6
o (degrees) @3 (C=C2-C3-F) o4 (C==C2-C4-F) F (mdyn
ss
GG
0.0 0.0
118.6 118.6
115.0 -115.0
0.2 118.0
0.070 0.070 --0.009
0.071 0.070 4.022
0.051 0.069 -0.075
GG”
GS
A rad-’
a’E/ao: a’E/ao: a’E/a@,ao,
0.051 0.051 -0.007
form did not correspond to an energy minimum for this molecule according to our calculations. The point on the broken curve at 80” is not a minimum on the potential surface. The values obtained for the torsion angles (@) and torsional force constants (F) are set out in Table 7. When the electrostatic terms were neglected in the calculations, the AE value increased to 1.8 kcal mol-‘. The minimum for GS was moved to o, = 49.8” and & = 12.4”. The potential was quite flat in this region. 3-Fluoro-1
-butene
Results for H,C=CH-HCF-CH3 are shown in Fig. 8. The most stable conformer according to our calculations, is GH with F eclipsing the double bond. However, GX with an H atom eclipsing the double bond is only 0.1 kcal mol-’ higher in energy. The third conformer (SM) is about 1.0 kcal
E
SS
G-5
.. xx
52
x
x
..
x
._ --v x
\
,.
5-t 4
x‘
?=t GG
Fig. 7. (A) cis-XH,C-HC=CH-CH,X. X = fluorine. Possible conformations of &s-1,4difluoro-2-butene. In the form SS the fragments X-Cl-C2==C3 and X-C4-C3=C2 are both planar (syn), corresponding to torsion angles 0,(X-Cl-C=C) = 0” and 0,(X-C4C=C) = 0”. The form GS has @, = 90” and 9, = 0”, and for GG $+ = o4 * 90”) while for GG” o1 * 90” and o4 = 90”. The symmetries are C,,, C,, C, and C, for the conformers SS, GS, GG and GG” respectively. (B) cis-XH,C-HC=CH-CH,X. X = fluorine. Torsional potential curves E (kcal mol-I) for cis-1,4-difluoro-2-butene. The conformers and their names (SS, GS, GG and GG”) are explained in (A). The transition form AS to C, symmetry. The unbroken curve (-) has 0, = 180” and o4 = 0” corresponding was obtained with 0, = o4 = o, and the broken curve (- ---) with o, = o and o4 = -0, while the dotted (. . . .) curve @, = 0 and o4 = 0”. TABLE
7 GS
@ (degrees) @, (F--CC2=C3) o4 (C2=C3-C4-F)
86.2 -13.1
GG
89.6 89.6
F (mdyn A rad”)
a’E/a@; azEja@: a2E/a@, a@,
0.051 0.047 0.036
0.029 0.029 0.023
mol-’ less stable than GH. The barrier may be taken from the potential curve terms were neglected, GH and SM were than GX, respectively. Calculated values force constants F are shown in Table 8.
heights separating the conformers in Fig. 8. When the electrostatic 0.1 and 0.8 kcal mol-’ less stable of the torsion angles and torsional
4-Fluoro-1 -butene Conformational drawings for HzC=CH-CHz-CH2F are shown in Fig. 9A. There are five conformers corresponding to well-defined minima of the potential curves in Fig. 9B. The calculated conformational energies (kcal mol-‘) relative to the energy of GG became: 0.16, 0.67, 1.66 and 2.13 for SG, GG*, GA and SA respectively. According to these calculated energies
293
0
30
60
90
120
150
180
Fig. 8. H,C=CH-XCH-CH, . M = CH, and X = fluorine. Torsional potential curves E (kcal mol-‘) for 3-fluoro-l-butene. The torsion angles o,(Cl=C2+3-X) and o4 (C2--C3C4-H) are in degrees. The conformers have the following values of o3 : 2.8 60” for these (SM), -120.8 (GX) and +116.5 (GH). The values of o4 are approximately conformers. Thus, in SM the CH, group eclipses the C=C bond; in GX and GH the Hatom and the X-atom eclipse the C=C bond respectively. The fragment C-XCH-CH, has a nearly staggered arrangement in each of these conformations. The unbroken curve was calculated with the restriction that o3 = +o and @4 = 60”, while the dotted curve =60”. corresponds to oa =-and@, TABLE
8 GX
GH
SM
-120.8 59.8
+116.5 60.2
+2.8 59.4
0.060 0.098 4.002
0.057 0.098 -0.002
0.068 0.154 +0.043
o (degrees)
@p(C=c--c---F) ~4 (C-C-C-H)
F (mdyn
A rad-‘)
a’E/a@: a’E/a@: aWa@,
a@,
the composition at 23” is: 46% of GG, 35% SG, 15% GG*, 3% of GA and 1% of SA. The values obtained for the torsion angles (4) and torsional force constants (F) are shown in Table 9. CONCLUDING REMARKS A summary of the conformers and the conformational energy differences within the fluoro compounds as well as a comparison with the corresponding chloro [3] and bromo [4] compounds are shown in Table 10. The chloro and bromo compounds possess the same conformers and the same order of stability. In contrast to this the fluoro compounds often have a reversed order of conformational stability. For the compounds cis-FHC=CH-CHzF and cis-FHzC-HC=CH-CH2F a greater number of conformations is stable than within the corresponding chloro and bromo compounds.
GA
GG
GG'
Fig. 9. (A) H,C=CH-CH,-CH,X. Possible conformations of H,C=CH-CH,-CH,X. SA and SG have the C atoms in a planar syn (S) arrangement. For GG, GA and GG* the C atoms have a gauche (G) arrangement. The second letter in a conformational name indicates whether X has an anti (A) or gauche (G) arrangement to the C=C bond. The torsion angles are o3 (C=C-C-C) and @a (C-C-C-X). Thus the values of the torsion angles (o:/o”,) are: SA(0/180), SG (O/60), GG (120/+60), GA (120/180) and GG* (120/-60). Only SA possesses C, symmetry. (B) H,C=CH-CH,--CH,X. X = fluorine. Torsional potential curves E (kcal mol-‘) for 4-fluoro-1-butene. The conformers and their names are explained in (A). The unbroken curve (-) was calculated with the restriction that o = o4 (C=C-C--C) and o, (C-C-C-X) = 120”. The broken curve (----) corresponds to the restriction b4 = -@ and o, = 120”, while for the dotted curve (.. . .) @4 = I#Jand o3 = 0”. TABLE
9 GG
GG*
GA
SG
SA
110.8 60.2
116.3 -60.5
117.1 181.0
0.8 62.0
0.0 180.0
0.055
0.058
0.061
0.093
0.073
0.236 0.081
0.150 0.043
o (degrees)
@, (C=c-c--o ~4 (C--C-‘-F)
F (mdyn .A rade2)
a'E/a& a’E/a& aZE/a@,a0,
0.126 -0.003
0.117 -0.007
0.093 -0.001
Values of the rotational barrier heights are found typically between 1 and 2 kcal mol-‘. However, for the transition SS -+ GS -+ GG in the molecule H2C=C(CH2F)z a barrier height less than 1 kcal mol-’ was obtained, while for the transition between enantiomeric GG forms in the same molecule a barrier as high as ‘7 kcal mol-’ was calculated. None of the molecules studied here possess a double minimum in the potential at syn or anti. If the atoms C=C--C--F or C=C-C-C have a planar anti arrangement, this is a transition form indicated by A or AS in the figures. The fluoro compounds show less variation in the structural parameter values than the analogous chloro and bromo compounds. However, the following typical differences between syn (S) and gauche (G) conformers
295 TABLE Summary
10 of conformers
and calculated
conformationalenergy
X = halogen
AEa
H,C=CH-CH,X cis-XHC=CH-CH,X tmns-XHC=CH-CH,X H,C=CX--CH,X tram-XH,C-HC=CH-CH, H,C=C(CH,)*H,X
G-S G-S G-S G-S G-S G-S GG”--GG GS-GG SS-GG GG”GG GS-GG SS-GG GH-GX SM-GX GG-GA SA-GA GG*-GA SG--GA
H,C=C(CH,X),
cis-XH,C-HC=CH-CH,X
H,C=CH-XCH-CH, H,C=CH-CH,CH,X
F +0.5 -0.2 +0.5 +0.8 +0.3 -0.6 +0.2 +0.7 +1.4 _c -0.6 +2.6 -0.1 +1.1 -1.7 +0.5 -1.0 -1.5
differences
AE in kcal mol-’
Br [41
Cl 131 -0.7
-0.8
_b
_b
4.4 +0.3 4.6 -1.4 +1.4 +2.0 +3.6 -0.4
4.3 +0.8 -0.7 -1.2 +1.6 +1.6 +3.a 4.1 _d
_e
4
+0.6 +o.g +0.4 +0.4 +1.1 +0.8
+O.B +2.2 +0.2 +0.6 +0.9 +0.9
=Energy difference AE between two conformers as indicated here; G (gauche) (syn). bOnly G is stable. ‘GG” not stable. dGS not stable. eSS not stable.
and S
exist (S-C): Ar(C-C) = 0.005 a Ar(C&-F) = 0.002 8, A(CCC) = 1” and A(C&F) = 2”. For gauche conformers the torsional force constants have values in the range 0.05-0.07 and the syn conformers have values in the range 0.040.06 mdyn a rad-’ . Neglecting the atomic excess charges in the calculations had a pronounced influence on some of the potential curves as shown in Figs. 1-3. ACKNOWLEDGEMENT
We are grateful to Norges Almenvitenskapelige support.
ForskningsrPd
for financial
REFERENCES 1 2 3 4 5 6 7 8
R. R. P. P. R. T. T. R.
St.&evik and 8. Thingstad, J. Mol. Struct., 106 (1984) 333. St$levik, J. Mol. Struct., 109 (1984) 397. J. Stavnebrekk and R. Snjlevik, J. Mol. Struct., 159 (1987) 153. J. Stavnebrekk and R. StQlevik, J. Mol. Struct., 160 (1987) 143. J. Abraham and R. StQlevik, Chem. Phys. Lett., 58 (1978) 622. Rydland, Thesis, University of Trondheim, AVH, Trondheim, 1981. Rydland and R. St&evik, J. Mol. Struct. (Theochem), 105 (1983) 15. T. Sanderson, Chemical Bonds and Bond Energy, Academic Press, New York,
19 76.
296 9 10 11 12 13
E. P. J. T. A.
Hirota, J. Chem. Phys., 42 (1965) 2071. Meakin, D. 0. Harris and E. Hirota, J. Chem. Phys., 51 (1969) 3775. R. Durig, M. Zhen and T. S. Little, J. Chem. Phys., 81 (1984) 4259. S. Little and J. R. Durig, J. Mol. Struct., 114 (1984) 385. D. English, L. H. Sharpen, K. W. Ewool, M. L. Strauss and D. 0.
Spectrosc.,
60 (1976)
210.
Harris, J. Mol.