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Barriers to Internal Rotation Determined by Microwave Spectroscopy
BARRIERS TO INTERNAL ROTATION DETERMINED BY MICROWAVE APPENDIX 9 I SPECTROSCOPY
Tables A9-1-6 compare internal barrier heights, conformations, and methyl group tilt angles for a number of molecules which have been studied by microwave spectroscopy. A more detailed compilation through 1962 has been given by Herschbach [737]. The effects of group substitutions in related series of molecules such as CH 3 CH 2 X, CH 3 COX, and C H 3 C H = C H X , where X may be hydrogen, a halogen, an alkyl group, etc., are determined by comparing barrier values given in the tables. For example, a general decrease in V3 might usually be expected from CH 3 CH 2 X to C H 3 C X = C H 2 to C H 3 C H = C H X to CH 3 COX ; however, exceptions can be expected. It is difficult to make correct empirical predictions of barrier heights because many factors can affect V(a). These include the bond length associated with the torsional motion, molecular symmetry, electronegativity of the substituents, and the type of bonding. The following comparisons provide examples. Substitution of silicon for carbon in ethane-type compounds results in a reduction of V3 which may be related to the increase in bond length. Extension through germanium and tin does not alter this reasoning. The reduction in barrier height from carbon to silicon is approximately a factor of two for ethane-type molecules and is even greater through ί-butane-type molecules. In the C—V group, the barrier increases as methyl groups are added to molecules like N H 3 , P H 3 , and AsH 3 . The rate of increase is greatest for the amines, on the order of one kilocalorie per methyl group. The phosphines show an analogous trend and limited data about the arsenes is in agreement. This trend is absent in the methyl-substituted silanes where the barriers for methylsilane and dimethylsilane are nearly identical ; the barrier in trimethylsilane is slightly larger. However, the C—VI compounds show a factor of two increase upon addition of a second methyl group to methyl alcohol or methyl mercaptan. Some very important reversals of this tendency come in the acetaldehydeacetone and propylene oxide-cfs-2,3-epoxybutane series. Addition of a 416
Barriers to Internal Rotation
417
second methyl group forces approximately a twofold barrier reduction. When an ethyl group is added, the barrier is reduced even more. It is interesting to note that both of these exceptions occur when the carbon atom to which the methyl group is attached is also bonded to an oxygen atom. A valuable by-product of the study of internal barriers is a determination of the minimum energy or equilibrium configuration of the top and frame. For the most fundamental case of ethane, the staggered form has a lower energy than the eclipsed form. It is evident that the forces which provide the hindering potential will dictate the favored configuration. One important conclusion is the general preference for the staggered form over the eclipsed form. The eclipsed form is the orientation of maximum torsional energy. If a complete quantum-mechanical calculation for the total energy of the molecule were done for the staggered and eclipsed forms, a simple difference would yield the barrier height directly ; however, this difference of two relatively large quantities would be subject to large percentage errors. The last few entries in Table A9-5 represent rotational isomers which have been studied by microwave spectroscopy. The gauche and eis forms are generally favored over the trans structure. In the close study of methyl group structure inherent in the investigation of internal rotation, it has been observed that in some molecules the axis of the methyl group is not colinear with the C—X bond about which the hindered rotation is executed. For C—O, C—N, and C—S bonds where the atom X has one or more sets of lone pair electrons, the axis of the methyl group appears to be tilted up toward the unshared electrons. This effect also has been observed for methyl groups bonded to unsaturated carbon atoms, e.g., ( C H 3 ) 2 C = 0 . The presence of a tilted methyl group is usually detected either from the internal rotation splittings, provided they are sensitive to the orientation of the top axis through the direction cosines, or from a study of species with partially deuterated methyl groups (CH 2 D and CHD 2 ) [722, 755]. A nonequivalent hybridization scheme has been applied to dimethyl ether ; by assuming the experimentally determined HCH angles to be orbital angles, sp3Al and sp2·81 hybridizations were calculated for the CH a and CH S bonds, a and s refer to positions in and out of the COC plane, respectively. It has been proposed that this hybridization and the apparent "bent bonds" in CH 3 XCH 3 molecules might be the result of a nonbonded repulsion between the two methyl groups [480], although the presence of lone pair or π electrons seems to be prerequisite. The existence of a "bent bond" would appear to have a relatively strong effect on the barrier. In the following tables error limits are not listed and may be obtained from the appropriate references.
418
Appendix 9
TABLE A9-1 BARRIERS TO INTERNAL ROTATION ABOUT CH 3 —C BONDS
Molecule
Formula
K(cal/mole)
Ethyl fluoride 1,1-Difluoroethane Methyl fluoroform Ethyl chloride Ethyl bromide Ethyl iodide Ethyl cyanide Ethyl alcohol Propane Ethyl silane ds-Propionaldehyde irans-Propyl fluoride gauche-Propyl fluoride Isobutane ί-Butyl fluoride Acetaldehyde Acetyl fluoride Acetyl chloride Acetyl bromide Acetyl cyanide Acetic acid Acetone Methyl vinyl ketone ds-Butanone Propylene irans-Fluoropropene ds-Fluoropropene 2-Chloropropene ds-Chloropropene 2-Fluoropropene irans-Crotononitrile ds-Crotononitrile Methyl ketene Methyl aliène Isobutylene Propylene oxide
CH 3 CH 2 F CH 3 CHF 2 CH3CF3 CH 3 CH 2 C1 CH 3 CH 2 Br CH 3 CH 2 I CH 3 CH 2 CN CH 3 CH 2 OH CH 3 CH 2 CH 3 CH 3 CH 2 SiH 3 CH 3 CH 2 CHO CH 3 CH 2 CH 2 F CH 3 CH 2 CH 2 F (CH 3 ) 3 CH (CH 3 ) 3 CF CH 3 CHO CH 3 COF CH 3 COCl CH 3 COBr CH 3 COCN CH 3 COOH (CH 3 ) 2 CO CH 3 COCHCH 2 CH 3 COC 2 H 5 CH3CH=CH2 CH3CH=CHF CH3CH=CHF CH 3 CC1=CH 2 CH 3 CH=CHC1 CH3CF=CH2 CH3CH=CHCN CH3CH=CHCN CH3CH=C=0 CH3CH=C=CH2 (CH 3 ) 2 C=CH 2 CH3CH-CH2
3310 3180 3500 3685 3567 3220 3050 772 3555 2650 2280 2690 2670 3900 4300 1167 1041 1296 1305 1270 497 778 1250 500 1978 2200 1057 2671 622 2432 >2100 1400 1200 1589 2210 2560
ds-2,3-Epoxybutane
O CH3CH-CHCH3
1607
l-Chloro-2-butyne Methylsilylacetylene 1 -Trifluorobutyne-2
\ O / CH 3 C^CCH 2 C1 CH3C^CSiH3 CH3C^CCF3
<100 <3 <300
References [659] [659] [621] [1578] [702] [1483] [700] [743a] [755b] [737] [1633] [725] [725] [691] [691] [673,698] [1466] [15391 [14891 [1455] [683] [705, 755] [1675] [737] [676] [681] [1549] [758] [1632] [1467] [1491] [733] [1545] [677] [1531] [682] [723] [701] [752] [737]
Barriers to Internal Rotation
419
Table A9-1 (continued) Molecule
Formula
Propylene sulfide
CH3CH-CH2
i-Butyl acetylene i-Butyl cyanide p-Fluorotoluene
(CH3)3CCEECH (CH 3 ) 3 CCN CH 3 C 6 H 4 F
\ S/
F(cal/mole)
References
3240
[1591]
-4000 -4000 13.82
[1572] [1572] [755al
TABLE A9-2 BARRIERS TO INTERNAL ROTATION ABOUT IV-- IV BONDS OTHER THAN CARBON—CARBON
Molecule Methylsilane Methylfluorosilane Methyldifluorosilane Methyltrifluorosilane Dimethylsilane Trimethylsilane Ethylsilane Chloromethylsilane Vinylsilane Methylgermane Methylstannane Disilanyl fluoride
Formula CH 3 SiH 3 CH 3 SiFH 2 CH 3 SiF 2 H CH 3 SiF 3 (CH 3 ) 2 SiH 2 (CH 3 ) 3 SiH C 2 H 5 SiH 3 CH 2 ClSiH 3 CH2=CHSiH3 CH 3 GeH 3 CH 3 SnH 3 SiH 3 SiH 2 F
F(cal/mole) 1670 1559 1255 1200 1647 1830 1980 2550 1500 1239 650 1048
References [674,698] [1433] [1436] [1176] [721] [1506] [737] [1579] [1535] [1456] [1516] [1699b]
TABLE A9-3 BARRIERS TO INTERNAL ROTATION ABOUT IV—III AND IV—V BONDS
Molecule Methylamine Dimethylamine Trimethylamine Methylphosphine Dimethylphosphine Trimethylphosphine Methyldifluoroarsine Trimethylarsine Nitromethane Methylisocyanate Trifluoronitromethane Methylazide Methylborondifluonde
Formula CH 3 NH 2 (CH 3 ) 2 NH (CH 3 ) 3 N CH 3 PH 2 (CH 3 ) 2 PH (CH 3 ) 3 P CH 3 AsF 2 (CH 3 ) 3 As CH3N02 CH 3 NCO CF3N02 CH 3 N 3 CH 3 BF 2
K(cal/mole)
References
1980 3300 4400 1960 2200 2600 1332 1500-2500 6.03 49 74.4 714 13.77
[786] [816] [690] [7191 [616] [1691] [15711 [1457] [665] [1594] [757] [1703] [679]
420
Appendix 9 TABLE A9-4 BARRIERS TO INTERNAL ROTATION ABOUT IV—VI BONDS
Molecule Methyl alcohol Methyl hypochlorite Methyl formate Methyl nitrate Dimethyl ether Ethylmethyl ether Methyl mercaptan Dimethylsulfide Dimethyldisulfide Methyl thiocyanate
Formula CH3OH CH3OCI CH3OOCH CH3ON02 (CH 3 ) 2 0 C 2 H 5 OCH 3 CH 3 SH (CH 3 ) 2 S (CH 3 ) 2 S 2 CH 3 SCN
F(cal/mole)
References
1070 3060 1190 2321 2720 2530 1268 2118 1600 1590
[1235] [1660] [1444] [1519] [6991 [737| [1487] [722] [756a] [754]
TABLE A9-5 MINIMUM ENERGY CONFIGURATIONS
Molecule CH 3 CH 2 F CH 3 CH 2 C1 CH3S1H3 CH 3 SiH 2 F CH 3 SiHF 2 CH 2 ClSiH 3 CH 3 GeH 3 CH3CHO CH3COF CH3COCI CH3COCN (CH 3 ) 3 CH (CH 3 ) 3 N (CH 3 ) 3 SiH (CH 3 ) 2 0 (CH 3 ) 2 S (CH 3 ) 2 CH 2 (CH 3 ) 2 C=CH 2 (CH 3 ) 2 NH CH3CH=CH2 SiH3CH=CH2
Configuration Staggered Staggered Staggered Staggered Staggered Staggered Staggered Methyl group eclipses oxygen and staggers hydrogen Methyl group eclipses oxygen and staggers fluorine Methyl group eclipses oxygen and staggers chlorine Methyl group eclipses oxygen and staggers cyanide group Each methyl group staggered with respect to methine group Each methyl group staggered with respect to lone pair Each methyl group staggered with respect to Siline group Each methyl group staggered with respect to opposite C = 0 bond Each methyl group staggered with respect to opposite C = S bond Each methyl group staggered with respect to méthylène group Methyl groups staggered with respect to the line colinear with the double bond Methyl groups stagger the C—N—C plane opposite from the N—H group Methyl group eclipses double bond and staggers methine H Silyl group staggered with respect to methine group
References [1475] [654] [674] [1433| [1436] [1579] [1456] [673] [1466] [1539] [1455] [881 ] [1506] [6991 [722] [1493] [1531] [816] [687] [1535]
Barriers to Internal Rotation
421
Table A9-5 (continued) Molecule
Configuration
CH300CH C 6 H 5 OH NH2NH2 NF2NF2 H202 CH 3 CH 2 CH 2 F CH 3 CH 2 CH 2 CN CH2FCH=CH2
References
Methyl group staggered with respect to formyl group Planar Dihedral angle of 90° Dihedral angle of 65° Dihedral angle of 120° Gauche configuration of CH 3 and F slightly more stable than trans Gauche configuration of F and CN slightly more stable than trans Cis configuration of F and C = C H 2 slightly more stable than trans
[1444] [711] [805] [1458] [732] [725] [726] [750]
TABLE A9-6 SOME APPROXIMATE METHYL GROUP TILT ANGLES
Molecule
Bond
CH30H (CH 3 ) 2 0 CH3OOCH CH 3 SH (CH 3 ) 2 S CH 3 NH 2 (CH 3 ) 2 CO CH3COCI CH3COCN SiH3CH=CH2
c-o c-o c-o c-s c-s C-N
c-c c-c c-c
Si-C
Angle -5° -5° -6° 2.5° 2.5° -3° 1.3° 1.6° 2.0° 1.8°
References [6681 [480,699] [1444] [1487] [722] [7931 [755] [7551 [755| [1535]
Tables A9-1-6 compare internal barrier heights, conformations, and methyl group tilt angles for a number of molecules which have been studied by microwave spectroscopy. A more detailed compilation through 1962 has been given by Herschbach [737]. The effects of group substitutions in related series of molecules such as CH 3 CH 2 X, CH 3 COX, and C H 3 C H = C H X , where X may be hydrogen, a halogen, an alkyl group, etc., are determined by comparing barrier values given in the tables. For example, a general decrease in V3 might usually be expected from CH 3 CH 2 X to C H 3 C X = C H 2 to C H 3 C H = C H X to CH 3 COX ; however, exceptions can be expected. It is difficult to make correct empirical predictions of barrier heights because many factors can affect V(a). These include the bond length associated with the torsional motion, molecular symmetry, electronegativity of the substituents, and the type of bonding. The following comparisons provide examples. Substitution of silicon for carbon in ethane-type compounds results in a reduction of V3 which may be related to the increase in bond length. Extension through germanium and tin does not alter this reasoning. The reduction in barrier height from carbon to silicon is approximately a factor of two for ethane-type molecules and is even greater through ί-butane-type molecules. In the C—V group, the barrier increases as methyl groups are added to molecules like N H 3 , P H 3 , and AsH 3 . The rate of increase is greatest for the amines, on the order of one kilocalorie per methyl group. The phosphines show an analogous trend and limited data about the arsenes is in agreement. This trend is absent in the methyl-substituted silanes where the barriers for methylsilane and dimethylsilane are nearly identical ; the barrier in trimethylsilane is slightly larger. However, the C—VI compounds show a factor of two increase upon addition of a second methyl group to methyl alcohol or methyl mercaptan. Some very important reversals of this tendency come in the acetaldehydeacetone and propylene oxide-cfs-2,3-epoxybutane series. Addition of a 416
Barriers to Internal Rotation
417
second methyl group forces approximately a twofold barrier reduction. When an ethyl group is added, the barrier is reduced even more. It is interesting to note that both of these exceptions occur when the carbon atom to which the methyl group is attached is also bonded to an oxygen atom. A valuable by-product of the study of internal barriers is a determination of the minimum energy or equilibrium configuration of the top and frame. For the most fundamental case of ethane, the staggered form has a lower energy than the eclipsed form. It is evident that the forces which provide the hindering potential will dictate the favored configuration. One important conclusion is the general preference for the staggered form over the eclipsed form. The eclipsed form is the orientation of maximum torsional energy. If a complete quantum-mechanical calculation for the total energy of the molecule were done for the staggered and eclipsed forms, a simple difference would yield the barrier height directly ; however, this difference of two relatively large quantities would be subject to large percentage errors. The last few entries in Table A9-5 represent rotational isomers which have been studied by microwave spectroscopy. The gauche and eis forms are generally favored over the trans structure. In the close study of methyl group structure inherent in the investigation of internal rotation, it has been observed that in some molecules the axis of the methyl group is not colinear with the C—X bond about which the hindered rotation is executed. For C—O, C—N, and C—S bonds where the atom X has one or more sets of lone pair electrons, the axis of the methyl group appears to be tilted up toward the unshared electrons. This effect also has been observed for methyl groups bonded to unsaturated carbon atoms, e.g., ( C H 3 ) 2 C = 0 . The presence of a tilted methyl group is usually detected either from the internal rotation splittings, provided they are sensitive to the orientation of the top axis through the direction cosines, or from a study of species with partially deuterated methyl groups (CH 2 D and CHD 2 ) [722, 755]. A nonequivalent hybridization scheme has been applied to dimethyl ether ; by assuming the experimentally determined HCH angles to be orbital angles, sp3Al and sp2·81 hybridizations were calculated for the CH a and CH S bonds, a and s refer to positions in and out of the COC plane, respectively. It has been proposed that this hybridization and the apparent "bent bonds" in CH 3 XCH 3 molecules might be the result of a nonbonded repulsion between the two methyl groups [480], although the presence of lone pair or π electrons seems to be prerequisite. The existence of a "bent bond" would appear to have a relatively strong effect on the barrier. In the following tables error limits are not listed and may be obtained from the appropriate references.
418
Appendix 9
TABLE A9-1 BARRIERS TO INTERNAL ROTATION ABOUT CH 3 —C BONDS
Molecule
Formula
K(cal/mole)
Ethyl fluoride 1,1-Difluoroethane Methyl fluoroform Ethyl chloride Ethyl bromide Ethyl iodide Ethyl cyanide Ethyl alcohol Propane Ethyl silane ds-Propionaldehyde irans-Propyl fluoride gauche-Propyl fluoride Isobutane ί-Butyl fluoride Acetaldehyde Acetyl fluoride Acetyl chloride Acetyl bromide Acetyl cyanide Acetic acid Acetone Methyl vinyl ketone ds-Butanone Propylene irans-Fluoropropene ds-Fluoropropene 2-Chloropropene ds-Chloropropene 2-Fluoropropene irans-Crotononitrile ds-Crotononitrile Methyl ketene Methyl aliène Isobutylene Propylene oxide
CH 3 CH 2 F CH 3 CHF 2 CH3CF3 CH 3 CH 2 C1 CH 3 CH 2 Br CH 3 CH 2 I CH 3 CH 2 CN CH 3 CH 2 OH CH 3 CH 2 CH 3 CH 3 CH 2 SiH 3 CH 3 CH 2 CHO CH 3 CH 2 CH 2 F CH 3 CH 2 CH 2 F (CH 3 ) 3 CH (CH 3 ) 3 CF CH 3 CHO CH 3 COF CH 3 COCl CH 3 COBr CH 3 COCN CH 3 COOH (CH 3 ) 2 CO CH 3 COCHCH 2 CH 3 COC 2 H 5 CH3CH=CH2 CH3CH=CHF CH3CH=CHF CH 3 CC1=CH 2 CH 3 CH=CHC1 CH3CF=CH2 CH3CH=CHCN CH3CH=CHCN CH3CH=C=0 CH3CH=C=CH2 (CH 3 ) 2 C=CH 2 CH3CH-CH2
3310 3180 3500 3685 3567 3220 3050 772 3555 2650 2280 2690 2670 3900 4300 1167 1041 1296 1305 1270 497 778 1250 500 1978 2200 1057 2671 622 2432 >2100 1400 1200 1589 2210 2560
ds-2,3-Epoxybutane
O CH3CH-CHCH3
1607
l-Chloro-2-butyne Methylsilylacetylene 1 -Trifluorobutyne-2
\ O / CH 3 C^CCH 2 C1 CH3C^CSiH3 CH3C^CCF3
<100 <3 <300
References [659] [659] [621] [1578] [702] [1483] [700] [743a] [755b] [737] [1633] [725] [725] [691] [691] [673,698] [1466] [15391 [14891 [1455] [683] [705, 755] [1675] [737] [676] [681] [1549] [758] [1632] [1467] [1491] [733] [1545] [677] [1531] [682] [723] [701] [752] [737]
Barriers to Internal Rotation
419
Table A9-1 (continued) Molecule
Formula
Propylene sulfide
CH3CH-CH2
i-Butyl acetylene i-Butyl cyanide p-Fluorotoluene
(CH3)3CCEECH (CH 3 ) 3 CCN CH 3 C 6 H 4 F
\ S/
F(cal/mole)
References
3240
[1591]
-4000 -4000 13.82
[1572] [1572] [755al
TABLE A9-2 BARRIERS TO INTERNAL ROTATION ABOUT IV-- IV BONDS OTHER THAN CARBON—CARBON
Molecule Methylsilane Methylfluorosilane Methyldifluorosilane Methyltrifluorosilane Dimethylsilane Trimethylsilane Ethylsilane Chloromethylsilane Vinylsilane Methylgermane Methylstannane Disilanyl fluoride
Formula CH 3 SiH 3 CH 3 SiFH 2 CH 3 SiF 2 H CH 3 SiF 3 (CH 3 ) 2 SiH 2 (CH 3 ) 3 SiH C 2 H 5 SiH 3 CH 2 ClSiH 3 CH2=CHSiH3 CH 3 GeH 3 CH 3 SnH 3 SiH 3 SiH 2 F
F(cal/mole) 1670 1559 1255 1200 1647 1830 1980 2550 1500 1239 650 1048
References [674,698] [1433] [1436] [1176] [721] [1506] [737] [1579] [1535] [1456] [1516] [1699b]
TABLE A9-3 BARRIERS TO INTERNAL ROTATION ABOUT IV—III AND IV—V BONDS
Molecule Methylamine Dimethylamine Trimethylamine Methylphosphine Dimethylphosphine Trimethylphosphine Methyldifluoroarsine Trimethylarsine Nitromethane Methylisocyanate Trifluoronitromethane Methylazide Methylborondifluonde
Formula CH 3 NH 2 (CH 3 ) 2 NH (CH 3 ) 3 N CH 3 PH 2 (CH 3 ) 2 PH (CH 3 ) 3 P CH 3 AsF 2 (CH 3 ) 3 As CH3N02 CH 3 NCO CF3N02 CH 3 N 3 CH 3 BF 2
K(cal/mole)
References
1980 3300 4400 1960 2200 2600 1332 1500-2500 6.03 49 74.4 714 13.77
[786] [816] [690] [7191 [616] [1691] [15711 [1457] [665] [1594] [757] [1703] [679]
420
Appendix 9 TABLE A9-4 BARRIERS TO INTERNAL ROTATION ABOUT IV—VI BONDS
Molecule Methyl alcohol Methyl hypochlorite Methyl formate Methyl nitrate Dimethyl ether Ethylmethyl ether Methyl mercaptan Dimethylsulfide Dimethyldisulfide Methyl thiocyanate
Formula CH3OH CH3OCI CH3OOCH CH3ON02 (CH 3 ) 2 0 C 2 H 5 OCH 3 CH 3 SH (CH 3 ) 2 S (CH 3 ) 2 S 2 CH 3 SCN
F(cal/mole)
References
1070 3060 1190 2321 2720 2530 1268 2118 1600 1590
[1235] [1660] [1444] [1519] [6991 [737| [1487] [722] [756a] [754]
TABLE A9-5 MINIMUM ENERGY CONFIGURATIONS
Molecule CH 3 CH 2 F CH 3 CH 2 C1 CH3S1H3 CH 3 SiH 2 F CH 3 SiHF 2 CH 2 ClSiH 3 CH 3 GeH 3 CH3CHO CH3COF CH3COCI CH3COCN (CH 3 ) 3 CH (CH 3 ) 3 N (CH 3 ) 3 SiH (CH 3 ) 2 0 (CH 3 ) 2 S (CH 3 ) 2 CH 2 (CH 3 ) 2 C=CH 2 (CH 3 ) 2 NH CH3CH=CH2 SiH3CH=CH2
Configuration Staggered Staggered Staggered Staggered Staggered Staggered Staggered Methyl group eclipses oxygen and staggers hydrogen Methyl group eclipses oxygen and staggers fluorine Methyl group eclipses oxygen and staggers chlorine Methyl group eclipses oxygen and staggers cyanide group Each methyl group staggered with respect to methine group Each methyl group staggered with respect to lone pair Each methyl group staggered with respect to Siline group Each methyl group staggered with respect to opposite C = 0 bond Each methyl group staggered with respect to opposite C = S bond Each methyl group staggered with respect to méthylène group Methyl groups staggered with respect to the line colinear with the double bond Methyl groups stagger the C—N—C plane opposite from the N—H group Methyl group eclipses double bond and staggers methine H Silyl group staggered with respect to methine group
References [1475] [654] [674] [1433| [1436] [1579] [1456] [673] [1466] [1539] [1455] [881 ] [1506] [6991 [722] [1493] [1531] [816] [687] [1535]
Barriers to Internal Rotation
421
Table A9-5 (continued) Molecule
Configuration
CH300CH C 6 H 5 OH NH2NH2 NF2NF2 H202 CH 3 CH 2 CH 2 F CH 3 CH 2 CH 2 CN CH2FCH=CH2
References
Methyl group staggered with respect to formyl group Planar Dihedral angle of 90° Dihedral angle of 65° Dihedral angle of 120° Gauche configuration of CH 3 and F slightly more stable than trans Gauche configuration of F and CN slightly more stable than trans Cis configuration of F and C = C H 2 slightly more stable than trans
[1444] [711] [805] [1458] [732] [725] [726] [750]
TABLE A9-6 SOME APPROXIMATE METHYL GROUP TILT ANGLES
Molecule
Bond
CH30H (CH 3 ) 2 0 CH3OOCH CH 3 SH (CH 3 ) 2 S CH 3 NH 2 (CH 3 ) 2 CO CH3COCI CH3COCN SiH3CH=CH2
c-o c-o c-o c-s c-s C-N
c-c c-c c-c
Si-C
Angle -5° -5° -6° 2.5° 2.5° -3° 1.3° 1.6° 2.0° 1.8°
References [6681 [480,699] [1444] [1487] [722] [7931 [755] [7551 [755| [1535]