Conformation Determination of 2,4=Pentanediol by Nuclear Magnetic Resonance Tomi Fukuroi, Yuzuru Fujiwara, and Shizuo Fujiwara Department of Chemistry, Faculty of Science, The University of Tokyo, Tokyo, Japan Kiyoshi Fujii The Kurashiki Rayon Co., Kurashiki, Okayama, Japan Two diastereoisomeric meso and racemic 2,4-disu bstituted pentanes serve as model compounds of vinyl polymers. Conformations of the 2,4-pentanediols are determined quantitatively by NMR spectra analysis, in particular through the investigation of their solvent and temperature dependence. The TT conformation of the meso compound and the TG-(= G-T) conformation of the racemic compound are the most stable conformations. It is shown that intramolecular hydrogen bonding plays an important part in stabilizing conformations. Intermolecular interactions between solvents and 2,4-pentanediol in solution are discussed.
conformation which corresponds to a planar zigzag structure, and to some extent the G+G+conformation. However, the conformations of 2,4-pentanediol have not been definitely established and are qualitative in nature. The conformations of 2,4pentanediol are interesting in that they may form both intra- and intermolecular hydrogen bonds. This paper will furnish a more detailed analysis of the relative populations of its conformations, and also give some information concerning their interactions with solvents. EXPERIMENTAL
STEREOREGULAR ISOMERS of polyvinyl compounds in solid have been analyzed by X-ray and infrared absorption methods. High resolution NMR was first applied to the study of configurations of polystyrene in solution (I), and to the quantitative analysis of tacticities of polymethylmethacrylate (2,3). Since then this method has been widely used successfully for the determination of the relative composition of synthesized high polymers which are a mixture of stereoregular isomers. The usefulness of this method has been established in practice; however, atactic, syndiotactic, and isotactic structures of isomers have been determined not only by the NMR method. For example, stereoregularities were first determined in solid synthesized samples by X-ray or IR methods. Then these samples with known stereoregularities were used as reference compounds for NMR measurements. The NMR method has been widely adopted as a useful method for the determination of stereoregularities in high polymers in solution ; however, structural investigation cannot be carried out in detail for very high polymers. This is because the high resolution features of NMR spectra are usually missed with samples of high polymers, as molecular motion becomes restricted. Because the stereoregularities of any compound are important in that they signify the species of the molecule, a study of the subject is of significance in analytical chemistry. Model compounds have demonstrated their importance in the study of structures of stereoregularities of high polymers. The present authors have carried out a series of investigations in order to establish the relationship between the vicinal spin coupling constant, J , the chemical shift values (hereafter abbreviated as the NMR parameters), and the conformation of 2,4-pentanediol and its acetates, and 2,4,6-heptanetriol and its acetates (4-7). Several publications have appeared concerning the NMR study of conformations in model compounds of vinyl polymers-e.g., polyvinylchlorides (8-1 I), polystyrene (12), polyvinyl alcohol (IO,13, Z4), and polyvinyl acetate (14). Most disubstituted pentanes, such as chloro, bromo, cyano, acetoxy, and phenyl derivatives have similar conformationsLe., meso isomers exchange between TG+ and G-T conformations which correspond to a 31 helical structure portion of the polymers, and racemic isomers predominantly take the 7T
2,4-Pentanediol was separated into meso and racemic isomers by column chromatography (15). Carbon tetrachloride, chloroform, methylene chloride, pyridine, dimethyl sulfoxide (DMSO), and deuterium oxide were used as solvents. The concentrations of solutions were kept at 10% (w/v). NMR spectra were obtained with an HA-100 spectrometer (Varian Associates) operating at 100 MHz. Measurements were carried out over a temperature range of -20" to 100" C. Spectral analysis was performed according to the previously reported method (1). RESULTS
An NMR spectrum of 2,4-pentanediol consists of a doublet (methyl protons), 12 asymmetric lines (meso methylene protons) or 4 symmetric lines (racemic methylene protons), 6 symmetric lines with further fine ' structures (meso methine protons) or simple 6 symmetric lines (racemic methine pro-
(1) F. A. Bovey and G. V. D. Tiers, J. Polymer Sei., 38,73 (1959). (2) Zbid., 44, 173 (1960). (3) A. Nishoka, H. Watanabe, I. Yamaguchi, and H. Shimizu, Ibid.,45, 232 (1960). (4) Y. Fujiwara, and S . Fujiwara, Bull. Chem. Soc. Japan, 37, 1005 (1964). (5) K. Fujii, Y. Fujiwara, and S . Fujiwara, Makro Molekulare Chemie, 89, 278 (1965). (6) Y. Fujiwara, S. Fujiwara, and K. Fujii, J. Polymer Sei., A-I, 4, 257 (1966). (7) S. Fujiwara, Y. Fujiwara, K. Fujii, and T. Fukuroi, J. Mol. Spectry., 19, 294 (1966). (8) D. Doskocilova, and B. Schneider, Collection Czech. Chem. Commun, 29,2290(1964). (9) D. Doskocilovh, J. Stokv, B. Schneider, H. Pivcova, M. Kolinsky, J. Petranek, and D. Lim, IUPAC Symposium, Prague (1965). (10) P. E. McMahon and W. C . Tincher, J. Mol. Spectry., 15, 180 (1965). (11) T. Shimanouchi, M. Tasumi, and Y . Abe, Makro Moleculare Chemie, 86, 43 (1965). (12) F. A. Bovey, F. P. Hood 111, E. W. Anderson, and L. C.Snyder, J. Chem. Phys., 42, 3900 (1965). (13) H. Buc, Ann. Chim., 8,431 (1963). (14) D. Doskocilovh, J. Stokr, E. Votavovl, B. Schneider, and D. Lim, ZUPAC Symposium, Prague (1965). (15) M.Shiraki and E. Nagai, J. Chem. SOC.Japan, 81,976 (1960). VOL 40, NO. 6, MAY 1960
e
879
. . . . I . . ' .
,"',".'
. . " I . " '
~ , ' , , " , ' 1 , , ' , ' , , , ,
x , a , , , . , , : , , , , , ' , , ,
300
200
. . . . . . . . . . . . , . , .. . . . . . . . . . . . , , , , , , , , l i l , , , , l , , , ,
, ' , ' , , , " " ,
I
>-H+
1w
0 CPS
+---.-----)
100 H r >
'
1
9
*
I
I
I
* , . . I . . . .
l
l
,
r
t
,
....
1
0
I
. . . . . . . . . .. .. .. . . . . . . . .. . .. .- . . .
I I t I C , J , ' , l I
I . . . .
....
I
.
,
.
.
....
, , .
I . . . .
I . . . .
I
.
.
.
.
, , , , , , , , ,
....
I
.
.
.
.
,
I
,
...,
Figure 1. (a) NMR spectrum of meso 2,4-pentanediol in pyridine solution tons) and a single line (hydroxyl protons). The spectra obtained in pyridine solution are shown in Figures la and b. It is seen that the chemical shifts of functional groups depend greatly on configuration and solvent. Methylene and methine protons of the meso compound show an especially large solvent dependence as seen in Figures 2 and 3, whereas those of the racemic compound do not. A detailed investigation of the methylene part of the spectra has been carried out as follows. Solvent Dependence. Twelve lines are observed for the methylene protons of the meso compound. They are divided into three groups, each of which has a quartet structure as shown in Figure 2. The - splitting - between the inner pair and the outer pair (ex. AB, CD) corresponds to the geminal spin coupling constant of the methylene protons, and the splitting of the inner pair (=) corresponds to a modification of the chemical shift between methylene protons, K and L, v K L , by the vicinal spin coupling constant of methylenemethine protons. These asymmetric spectra are reversed with regard to field if the sign of the chemical shift is reversed, or if the relative magnitudes of the vicinal spin coupling constants between methylene and methine protons are reversedLe., J A K 4J'AL,J A L + J'AK. Therefore, the same patterns, which are analyzed to give the parameters shown in Table 11, can be obtained from the parameters which are produced by interchanging JAK and JAL,JBK and JBL,and by reversing the sign of v K L simultaneously. However, this procedure results in changing the designation by replacing K by L and L by K. Therefore, these new parameters are essentially the same as those in Table 11, where the sign of the chemical 880
ANALYTICAL CHEMISTRY
shift is decided upon after taking JAK as the smaller coupling constant for the vicinal coupling between methylene and methine protons, and JALas the larger. For methylene protons of meso compounds in deuterium oxide, dimethylsulfoxide, pyridine, and carbon tetrachloride solutions, the splitting of the low field group is larger, and that of the high field group smaller than that of the central one. This indicates that the methylene chemical shift, Y K L , has a positive sign. On the other hand, in methylene chloride, chloroform, and dilute carbon tetrachloride solutions, the splittings of the three groups of methylene protons increase with field-Le., Y K L has a negative sign. In other words, this result suggests that the K proton appears at lower field than the L protons. As will be shown later, the meso form prefers the planar zigzag (TT) structure to the helical structure. Hence, in this structure, the K proton which is in trans position with respect to the alcoholic group is more shielded than the L proton in the gauche position. This finding is consistent with the general trend in alcoholic compounds. Concerning the vicinal spin coupling constant between methylene and methine protons in meso compounds, JAK is equal to J B K , and JAL is equal to J B L in all solvents. These relations indicate that the molecule has a plane of symmetry encompassing the methylene protons. For racemic compounds, the spectrum of methylene protons is a triplet whose center line splits into two. Because the spectra are symmetric, the sign of the chemical shift between the methylene protons cannot be decided. However, this is not a problem because the value is zero, as indicated by the symmetry of the spectra and the absence of splitting of the outer lines of methylene. The
1w
300
splitting of the central group is due to the difference between JAR and JAL.The relation among the vicinal spin coupling constants between methylene and methine protons is J A K = JBL and JAL = J B K in all solvents. This shows that the part including methylene and methine protons has one rotational axis of symmetry through the methylene carbon and the midpoint of the methylene protons. The difference in spin coupling constants between methylene and methine, J AR J A L , depends on solvent for both the meso and racemic compounds. The s u m of the vicinal spin coupling constants between methylene and methine protons, J A R JAL, for the meso compound is always larger than that for the racemic one. The chemical shift of the alcoholic OH protons is always to lower field in the meso compound than in the racemic one. Results of analysis at 40" C are listed in Tables I and 11. Temperature Dependence. Both the geminal spin coupling constant of the methylene protons, J K L , and the vicinal spin coupling constant between methyl and methine protons, JAX, are temperature independent. However, the s u m and the difference of the methylene-methine proton spin coupling constants-2J and AJ, respectively-depend on temperature. Though the chemical shifts of functional groups also show a temperature dependence, in a way which is in accordance with stereoregularities and solvents, they are not discussed in this paper because they do not directly reflect the population among conformations. The temperature dependence of AJ, ZJ, and V E L are shown in Tables 111, IVYand V, respectively. The temperature dependence of AJ was affected by solvent and falls into two groups. One group consists of
-
carbon tetrachloride, chloroform, and methylene chloride solution; the other consists of pyridine, dimethylsulfoxide, and deuteriumoxide solutions. For the former, the range of change was larger in the meso compound than in the racemic one, and for the latter the tendency was reversed. 2 J decreased with temperature although the variation was small. The change in V K L with temperature occurred only in meso compounds, and the absolute value decreased linearly with temperature.
+
Table I. Chemical Shifts of 2,ePentanediol (units, ppm; temperature, 40" C ) Solvent Isomer CH1 CHz CH DzO meso 8.83 8.43 6.11 racemic 8.86 8.48 6.12 DMSO meso 8.88 8.53 6.16 racemic 8.87 8.57 6.16 8.69 8.28 5.76 racemic meso 8.64 8.21 5.54 CH2CI2 meso 8.84 8.51 6.03 racemic 8.80 8.46 5.93 CHClr meso 8.82 8.49 6.01 racemic 8.79 8.43 5.87 CClP meso 8.88 8.55 6.11 racemicb 8.84 8.53 6.01 Observed at 60" C. b Saturated solution.
Q
VOL 40, NO. 6, MAY 1968
OH 5.40 5.45 5.59 5.87 4.28 4.33 6.37 7.00 6.06 6.46 5.42 6.87
881
I
A
1 B
C
D
L
1
I
DMSO
I
1
CHClg
pyridine
20
Hz
Figure 2. Solvent dependence of methyleoe proton spectra of mea0 2,4-pentanediol
882
0
ANALYTICAL CHEMISTRY
I
C HC , l2
DMSO
CHCli
pyridine
CCL,
__r
20 Hz Figure 3. Solvent dependnce of methine proton spectra of mesO 2,4pentanediol
Table 11. N M R Parameters of 2,4Pentanediol (units, Hz at room temperature) Solvent
Isomer
JAR
JAL
JB K
JB L
JRL
YKL
AJ
ZJ
DIO
meso racemic
5.7 4.3
7.5 8.5
5.7 8.5
7.5 4.3
13.8 13.8a
15.3 0
1.8 4.2
13.2 12.8
DMSO
meso racemic
4.8 4.1
7.8 7.9
4.8 7.9
7.8 4.1
13.5 13.P
18.0 0
3.0 3.8
12.6 12.0
meso lW€llliC
3.8 3.7
9.0 8.2
3.8 8.2
9.0 3.7
13.7 13.7a
24.0 0
5.2 4.5
12.7 11.9
CHsClz
meso racemic
2.6 3.4
10.0 8.0
2.6 8.0
10.0 3.4
14.4 14.4.
-6.0 0
7.4 4.6
12.6 11.4
CHClr
meso raCemiC
2.8 3.4
9.8
2.8 8.2
9.8 3.4
14.0 14.00
-2.4 0
7.0 4.8
12.6 11.6
8.2
J: spin coupling constants. v : chemical shift A,? = ( J A K JAL( Z J = ( J A K J A L ~
-
+
a The JKL’Sfor the racemic isomer are not obtainable from the spectrum because of the smallness of for the mesoic form are used here as J K Lfor the racemic form.
VKL
and AJ. Hence, the values
VOL 40, NO. 6, MAY 1968
883
Table 111. (A) The Temperature Dependence of AJ Observed in meso 2,4-Pentanediol O C
1/T X 10'
CClr
CHClr
CHzClz
DMSO
Dz0
-44
4.37 4.22 4.01 3.88 -3 3.70 +7 3.37 18 3.44 29 3.31 38 3.21 3.18 41 46 3.13 51 3.09 56 3.04 62 2.98 65 2.96 74 2.88 84 2.80 2.72 94 2.65 104 114 2.58 Experimental error, f0.2 Hz Temperature accuracy, f1 O C
-36 -24 - 15
6.50 6.16 5.98 5.89 5.72
6.98 7.11 7.05 7.05 6.92 6.94 6.84 6.97 6.89 6.70 6.54 6.42
8.15 7.81 7.87 7.90 7.80 7.59 7.63 7.46 7.40 7.34 7.40 7.27 7.37
5.38 5.40 5.38 5.36 5.35 5.34 5.28 5.26 5.26 5.26 5.27 5.22 5.15 5.09 5.08
1.69
3.00
3.09 3.19 3.11 3.11 3.14
1.74 1.89 1.78 1.93 1.92 1.86 2.00 1.96 1.96
3.17
3.21 3.15 3.14
2.07 1.94
(B)The Temperature Dependence of AJ Observed in racemic 2,4-Pentanediol "C
1/T x 10'
-44 4.37 - 36 4.22 -24 4.01 - 15 3.88 -3 3.70 $7 3.57 18 3.44 29 3.31 38 3.21 41 3.18 46 3.13 51 3.09 56 3.04 62 2.98 65 2.96 74 2.88 84 2.80 94 2.72 104 2.65 114 2.58 Experimental error, f0.1 Hz Temperature accuracy, f1 C
CHCh 6.08 5.72 4.80 4.38 5.10 4.71 5.04 4.92 4.77 4.68
ANALYTICAL CHEMISTRY
5.40 5.09
5.04 4.80 4.77
4.50
4.18
4.92 4.80 4.48
4.50 4.39 3.96
4.61 4.38
4.55
The Population of Conformations. Nine conformations can be assumed for 2,4-pentanediol. Conformations and vicinal spin coupling constants are shown in Figures 4 and 5, where T symbolizes that the mutual location of the main chain is trans and G that it is gauche. Plus and minus signs are defined as a clockwise and a counterclockwise rotation about a carbon-carbon bond, respectively. For a meso compound, the TT,G+G-, and G-G+ conformations have planes of symmetry encompassing the methylene protons. Three pairs of conformations (TG+, G-T; G+T, TG-; and G+G+, G-G-) are mirror images of each other, but they have no symmetry element. Because the methylene protons HK and K L of this compound are in different magnetic environments in all conformations, a chemical shift between the methylene protons, v K L , is expected. The analyses of spectra show that v K L of meso compounds has a value which de884
DMSO
3.71 3.31
pends on solvent and temperature. This suggests that the population of conformations changes with the solvent and temperature. For a racemic compound, conformations 77',G+G+, and G-G- have two-fold axes. The other three pairs of conformations ( T G , G-T; G+T, TG+; and G-G+, G+G-) are identical if assignments of protons are changed A for B, B for A, K for L,and L for K . Therefore, the methylene protons IfK and K L in racemic compound are magnetically identical, so the chemical shift between methylene protons v K L should be zero. v K L of a racemic compound is shown to be zero in all solvents, and over the whole temperature range from the analyses of spectra. On the basis of symmetry of all conformations, it is suggested for the vicinal spin coupling constants between methylene and methine protons that JAK is equal to JBK,and JALequal to JaL for a meso compound,
Table IV. (A) The Temperature Dependence of ZJ Observed in meso 2,4Pentanediol O C
1/TX
lo8
CCl,
CHCli
CHICli
12.6
12.4 12.4 12.5 12.4 12.3 12.4 12.5 12.3 12.4 12.4 12.3 12.5 12.4
-44 - 36 -24
4.37 4.22 4.01 - 15 3.88 -3 3.70 +7 3.57 18 3.44 29 3.31 38 3.21 41 3.18 46 3.13 51 3.09 56 3.04 62 2.98 65 2.96 74 2.88 84 2.80 94 2.72 104 2.65 114 2.58 Experimental error, hO.1 Hz Temperature accuracy, f1 a C
12.6 12.4
12.4
12.4 12.5 12.4 12.4 12.3
12.4 12.5 12.5 12.5
DMSO
12.8 12.8 12.8 12.8 12.6 12.9 12.7 12.6 12.4 12.5 12.6 12.6 12.5 12.5 12.5
DpO
13.1
13.5 13.4 13.6 13.5 13.2 13.2 13.1
12.8
13.1
12.8 12.6 12.8 12.9 12.8 12.6
13.0 13.1
13.0
(B)The Temperature Dependence of ZJ Observed In racemic 2,4Pentanediol "C
1/Tx 10'
CHCls
12.1 11.6
12.1 11.4 11.4 11.4
11.4
12.0
11.5 11.3
12.1 11.7 11.7 11.4 11.9
11.5 11.2
11.5 11.7
11.2 11.3
12.1 11.7 11.9
11.2 11.6
and JAR is equal to JBL and JnL equal to JBK for a racemic compound. These relationships are seen in the experimental results. The vicinal spin coupling constants, and the chemical shift between methylene protons are observed as the population average of the characteristic values of each conformation under the condition of rapid exchange among conformations, if they depend mainly on intramolecular magnetic circumstances. They are given as e
DMSO 12.0
-44 4.37 -36 4.22 -24 4.01 - 15 3.88 -3 3.70 +7 3.57 18 3.44 29 3.31 38 3.21 41 3.18 46 3.13 51 3.09 56 3.04 62 2.98 65 2.96 74 2.88 84 2.80 94 2.74 104 2.65 114 2.58 Experimental error, fO.l Hz Temperature accuracy, =k 1 C
Jobs
CHzClz
z,xiJi
(1)
11.8
formation, respectively. The populations of the various conformations may be given as a function of the energy difference between the conformations. Thus X i may be approximately written as
X, = exp( - A E , / R T ) / Z , exp( -AEJRT)
(3)
-
where Ei = Ei Eo, Eo being the energy of the conformation taken as reference. The sum is taken over all important conformations. Therefore, AE{ and Xi can be obtained by an examination of the temperature dependence of AJ, ZJ, and VKL.
where X,, J,, and v( are the population, vicinal spin coupling constant, and methylene protons chemical shift of ith con-
However, the characteristic chemical shift values of each conformation are difficult to estimate. Because the chemical shift is affected not only by intramolecular,but also by intermolecular interaction, the difference in intermolecular interVOL 40, NO. 6, MAY I960
* 885
JAK iJAL = JEKiJG JBL'JT
JAK JT JAL JBK= J B L = J ~ JAL=JEK= JEL:JG
11
11
11
JAK=JBK:JBL=Jg JAL'JT
JAti =JAL JBL= Jo JBK=JT
JAKZJBK =JG SAL =JBL -J T
JAK = JWSJr JALiJELiJG
JAL=J~ JAK:J~;=JBL.J~
JAK: JAL= JSK=JBL:J G
JAKJT
Figure 4. Conformations of meso 2,Cpentanediol
Figure 5. Conformations of racemic 2,Cpentanediol
action for each conformation cannot be neglected. On the other hand, spin coupling constants are affected mainly by intramolecular interaction. The population of each conformation, therefore) can be properly calculated on the basis of Equation 1. Though the population of conformations can be calculated by either AJ or ZJ,AJ was used because of its larger dependence on the temperature. The following approximation, moreover, was set up: the vicinal spin coupling constant for the gauche form JQ, and the trans form JT are independent of temperature over the range of the experiments. This assumption may be reasonable because the
temperature dependence of vicinal coupling of methylmethine protons, and of geminal coupling of methylene protons, should be very small. If one wishes to obtain an accurate value, the temperature dependence of JQ and JT cannot be neglected, as torsional vibrations may produce a modest temperature dependence in opposite directions for JG and JT. Meso G-G+,G+G-,and racemic G-G+,G+G-conformations were neglected because of their high energy, for methyl groups come near each other in their conformations. The observed values of vicinal spin coupling constant J A r and JALfor the meso form are given by
Table V. The Temperature Dependence of "C
-44 - 36 - 24
- 15
1/TX
loa
4.37 4.22 4.01 3.88 3.70 3.57 3.44 3.31 3.21 3.18 3.13 3.09 3.04 2.98 2.96 2.88 2.80 2.72 2.65 2.58
-3 +7 18 29 38 41 46 51 56 62 65 74 84 94 104 114 Experimental error, fO.l Hz. Temperature accuracy, f1 c
886
ANALYTICAL CHEMISTRY
CClr
CHCI,
-3.3 -2.7 -2.2
3.6 3.6 3.6 3.6 3.6 3.6 3.6
-1.7 -1.8 -1.4 -1.1 -0.6 -0.1 +O. 3
V
~
Observed L in mesO 2,CPentanediol CHiCli
-6.4 -6.5 -6.3 -6.1 -5.9 -5.3 -5.5 -5.2 -5.0 -4.7 -4.6 -4.1 -3.5
DMSO
26.5 25.4 24.7 23.7 23.5 22.7 22.5 22.3 21.4 20.7 19.8 19.1 18.6 17.9 17.4
17.6 16.8 16.4 16.1 15.6 15.1 14.8 14.4 14.0
Dt0
16.1 15.9 15.1 15.7 14.8 15.3 14.6 14.3 14.3 14.0 13.6
1 1.0 c AVS
The s u m of
The range of measurement #
(a)
r
ZJ
(JdK)ob8
xi(JT f Jo)
and ( J A L ZJ, ) ~is ~written ~,
+ ~ X Z ( J+T Jo) f
i
(x4
ZJ
=
(JT
+ 10)- 6Je-x 1+
0,,5
+ ,-AEa/RT + 2e-AEa/RT + ze-AE1/RT
AEz/RT
2e-AEi/RT
:
0.0
= Eo-QETT. By analogy with the treatment for the meso compounds, the observed vicinal spin coupling constants for the racemic compound may then be written
1
2
3
9
4
-1/T
5
~‘10’
1 . a=2 b=c=3 , 2, a=l b,c m , 3. a = l b=2 c = 3 4. a = b = c = l , 5. a=b=c=0.5 , 6. a=b=O c=l 7 . a=O b=l e-2, 8. a=O b=c=l , 9. a=-1 b=c=l
measurement
A J = (Xi
E 1
2
1. a=b=c=l d m 2. a=b=c:Q5 d = f 3. a = l b=O c=2 d
-
E~*G* ETTsaR, EQ-G- Et7 = d R .
-
3
4
ZJ
00
+ + JQ)+ 2XJQ+ XdJT + = X~JT+ XAJT + Jo) f X@T + Jo) + 2Xdo JO)
(4)
(5)
where XI, Xa, X,, and x6 are the populations of the 77’, TG+ or G-T, G+T or TG-, G+G+ or G-G- conformations, respectively. The difference in the above expressions is (Xi
=
+ X4 - XE)(JT- Jo)
(Xi
+ X4 - X&J
(6)
1W can be written as a function of temperature; 1 + ,-AEs/RT
- ,-AEa/RT
AJ = 6J 1 + 2e-AEdRT + 2e-AE2/RT
e-AEi/RT
(JT f J Q )
&+a+
= EotT
E~e--Ett=bR,E~$-ETT=cR, R ; gas constant
(JAR)ob
=
AE1
Figure 6. (a) Typical examples of AJ/8J curves of a me80 form (6)Typical examples of AJ/8J curvw of a racemic form
-
- Xz + Xs)GJ = 1 - ,-AEi/RT
1+
I/T x103 b=-1 c = l d 00
X~JQ XAJT
&JT
+ ~,-AEI/RT
+ e-AEa/RT
+ ze-AEa/RT
+
2e-AEa/RT
+
e - AEd/RT (12)
5
4. a.0 5 . a=b=O c.1 d.2 6. a=O b = l c.2 d
w
6J 1 +
-
I
X~JQ f
where Xi, X2, Xs, Xs, and X7 are the populations of the TT, G+G+,G-T or TG-, TG+ or G +T, and G-G- conformations, respectively. Then,
The range of
0.0
=
(JT
Jff)
-
(4
+ + Xs) + Jo) + X7Jo (10) + ~ X J+ Q X~(JT+ + X~JQ (11)
(Jm)oba = XJTf X ~ Q (X3 (JAL)o~
ETC+-ETT=aR,ET*- - E T T = ~ R , E,$e+ ETT- cR R i gas constant
(JAL)ob.
-
-
-
1W
(8)
In the above equations, AEI = EQ-T ETT= ETo+ - ETT, A& = & t T - E T T = ETQ- EFT,and AE3 = EQ+o+ - ETT I
{Jd&)ob#
+ 3Jo)
and thus its relation to temperature can be given as follows:
J
T
(JAX)ob#
x6) (JT
(7)
- 6J X
,-AEi/RT
e-
+ ze-AEz/RT
- E T T , A&
- ETT=
AEa/RT
= EQ-T
ETQt
-
+ ze-AEa/RT
- EFT = E T Q -
ETT,
(13)
- EFT,
= &-a-
- ETT
From Equations 7 and 12, a A J / GJcurve was calculated as a function of 1/Twhere the energy difference between conformations was introduced as the temperature independent parameter. Steric energies of conformations were calculated assuming interactions characterized by Lennard-Jones “6-1 2” type potentials between nonbonded atoms and groups [See McMahon and Tincher (IO)]. On the other hand, Fujiyama and Shimanouchi (16) presented the main factor of the conformation energy as the electric dipole-dipole interaction between substituted groups. The latter consideration leads to the conclusion that the meso TT and the racemic TG-(= G-T) conformations have higher energy than all other conformations by about 2 4 kcal/mole because of shorter distance between two substituted groups. The electric dipole-dipole contribution to the conformation energy, however, is the same order of magnitude as the steric repulsive one. According to both treatments, moreover, the effect of hydrogen bonding in these compounds was not considered. In the case of 2,4-pentanediol, an intramolecular hydrogen bond can be formed for the me!o TT in order that the distance of OH groups is about 2.5A. An attraction by hydrogen bond formation is at work rather than steric repulsion or dipole-dipole repulsion in these conformations. Moreover, stabilization with intermolecular hydrogen bonding in
-
(16) T. Fujiyama and T. Shimanouchi, J . Chern. Phys., 39, 1138 (1963). VOL 40, NO. 6, MAY 1968
887
solution must be taken into consideration for other conformations. Therefore, the stabilities of conformations in which intramolecular hydrogen bonds may be formed, meso TT and racemic TG-( = G-T) conformations, remain unexplained in the above treatments. The energies of other conformations presented by McMahon and Tincher (IO),however, are assumed to be in proper order, but improper in magnitude, inasmuch as conformations may be stabilized by hydrogen bond formation with solvent, or electric dipole-dipole interaction in solution. With the further assumption that electric dipole-dipole interaction is dominant, and the steric energy of the molecule is negligibly small, the energies of these conformations are regarded as almost the same. From the above, the following conditions are imposed on parameters in Equations 7 and 12. For the energies of mesoic conformations: TG+ 6 TG-
5 G+G+ 4>
AEa
--
AJ 8.9 8.5
8.3 8.7
-60
-13
-13
-13
-1.4
-1.4
-1.4
-2.0
-7.6
-53
-16
-16
-16
-1.2
-1.2
-1.2
-1.6
-8.0
should be fast. The variation of the OH proton chemical shift indicates that the stabilization by hydrogen bonds may be 2 to 4 kcal/mole, so most of the OH protons are in a hydrogen bonding state. Only the TG+ (or G-T)conformation, however, mixes to a small extent with the TT conformation in CCl,, CHCls, and CH2C12 solutions. This suggests the existence of a monomer-dimer equilibrium through formation of solute-solute intermolecular hydrogen bonds. In pyridine, DMSO, and D 2 0solutions, the TG+(or G-T)conformation is predominant, though it is in a high energy state from the viewpoint of the steric interactions. The TG+ (or G-T) conformation corresponds to part of a 31 helical structure, in which the OH protons are most favorably situated to interact with other molecules. This indicates that in pyridine, DMSO, and DzOsolutions, the stabilization by solute-solvent intermolecular hydrogen bonds is larger than that by intramolecular ones, and this may offset the energy disadvantage of the main chain. On the other hand, for racemic compounds the TT conformation may be the most stable one, and the TG-,G-T; G+G+and TG+,G+T conformations may be less stable in that order from the viewpoint of the steric interactions. In CHCls and CH2C12 solutions, however, the TG-,G-T conformations are as stable as the 7T conformation. The TG- or G-T conformation corresponds to the 21 helical structure, and the two OH protons are so close (about 2.5 A) that an intramolecular hydrogen bond may be formed similar to that in the 'IT conformation for the meso compound. Therefore, intramolecular hydrogen bonds are formed in CHCls and CH2Cl2 solutions. The TT conformation of the racemic compound may be still stabilized by forming solutesolute intermolecular hydrogen bonds similar to the TG+ and G-T conformations of the meso compound. However, the results in Table VI show the small population for the IT
conformation. This may be due to the entropy of mixing. On the other hand, in pyridine and DMSO solutions the population of the TTconformation is about half, and those of the G+G+,TG+ (G+T)conformations are as large as that of the TG- (G-T)conformation. Therefore, the stabilization by solute-solvent intermolecular hydrogen bonds is larger than that of intramolecular ones in those solutions. Conversely, 2,4-pentanediol interaction with solvents is small in CCL, CHC13, and CHzC12 solutions, but large in pyridine, DMSO, and DzOsolutions. This corresponds to the fact that 2,4-pentanediolhas a large solubility in pyridine, DMSO, and DzO, but a small solubility in CC14,CHC13,and CHzCl2. Concerning the chemical shift of the OH protons, that for the racemic compound is at higher field than that for the meso compound in all solvents. Therefore, the stabilization by hydrogen bonds is larger in meso compounds than in racemic compounds. In conclusion, for meso-2,4-pentanediol, a planar zigzag structure-Le., the IT conformation-is stable. However, it interacts with solvents so easily that helical structures also become stabilized by solute-solvent intermolecular hydrogen bonding. Therefore, the populations of the conformations depend largely on solvent. For racemic 2,4-pentanediol, 21 helical structures which are stabilized by intramolecular hydrogen bonds are as stable as a planar zigzag structure. Because a planar zigzag structure interacts more easily with solvents, however, the population of the TT conformation increases as the dielectric constant of solvent becomes larger. RECEIVED for review December 29, 1967. Accepted January 10, 1968. Presented in part at the International Symposium on Macromolecular Chemistry, Tokyo, Japan, September 1966.
VOL 40, NO. 6, MAY 1968
a
889
