J. Phys. Chem. 1987, 91, 6139-6148 be ascribed to a reduction of the nephelauxetic effect upon reCrystal field effects owing to J mixingz3 placing HzO by would not result in a shift of all transitions. This shows that the substitution of H 2 0 by F results in a decrease of the covalency of the Eu3+-ligands bonding. (b) Whereas the crystal field splitting of the 7F2level is equal for [EuC2.2.ll3+ and [ E u C 2.2.1I3+-2F, that of the 7F1is much larger for [EuC2.2.1I3+-2F than for [EuC2.2.1I3+(-300 cm-' vs -100 6m-l: respectively). The 7F, splitting is known to indicate the magnitude of the electrostatic contribution to the crystal fieldsz3This is clearly much larger for [EuC2.2.1I3+-2F than [EuC2.2.1I3+ in agreement with the shift mentioned above and with the general expectation that the introduction of F will decrease the amount of covalency. As to the [EuC2.2.1I3+-F complex, its emission spectrum does not fit in between those of [EuC2.2.1I3+and [EuC2.2.1I3+-2F, except for the 0-0 transition. In fact, the center of gravity of the 0 - 1 transition for [EuC2.2.1I3+-F lies in between those for the [EuC2.2.1I3+ and [EuC2.2.ll3+-2F complexes, although it is clear that this time the singlet component is at longer wavelength, indicating a change of sign of the electrostatic component of the crystal field compared to the [EuC2.2.1I3+-2F species. The 0-2 transition shows two new components. It is therefore, difficult to determine the center of gravity, but it is well possible that this is situated in between the maxima of the 0-2 transition of the [ E u C 2.2. I ] 3+ and [EuC2.2. 1I3+-2F complexes. For the Tb compounds, besides the 7FM ground multiplet, the 5D4luminescent level can also split at low ~ y m m e t r i e smaking ,~~ difficult any elucidation of the molecular structure from the emission spectrum. Thus, we only note that for [TbC2.2.1I3+ the changes in the emission spectra on increasing F concentration are less dramatic than for [EuC2.2.1I3+,suggesting that there F.23924
(23) Holsa, J.; Leskela, T.; Leskela, M. Inorg. Chem. 1985, 24, 1539. (24) Albin, M.; Horrocks, W. Dew. Inorg. Chem. 1985, 24, 895. (25) Reisfeld, R. Srruct. Bonding (Berlin) 1975, 22, 123.
6139
is an increase in the band splitting (Le., a decrease in symmetry) in passing from [TbC2.2.1I3+ to [TbC2.2.1I3+-F and from the latter species to [TbC2.2.1I3+-2F. Such a different behavior of the Eu3+ and Tb3+ compounds is presumably related to the slightly different ability of the two cryptates to coordinate water Conclusions Perturbation of the absorption and luminescence properties of the [MC2.2.1I3+cryptates ( M = Eu3+ or Tb3+)by addition of F ions reveals that the F ions replace water molecules in the holes of the cryptate structure and are directly coordinated to M3+,thus limiting the radiationless decay of the excited state via coupling with the 0-H oscillators. In the case of M = Eu3+,coordination of F also shifts the LMCT excited states of the cryptate to higher energies, thus preventing the radiationless decay of the 5L6and SDostates via a low-lying, distorted LMCT level, and bringing to unity the value of the luminescence quantum yield of [ E u C 2.2.lI3+ in DzO. The results (i) confirm that LMCT excited states can play a substantial role in governing the luminescence properties of Eu3+ complexes, (ii) show that it is possible to modify the absorption and luminescence properties of lanthanide complexes by suitable ion-pair perturbation, and (iii) supply another example of elucidation of molecular structures by electronic absorption and luminescence spectra and lifetimes.
Acknowledgment. We thank Prof. G. Blasse for stimulating discussions on the interpretation of the emission spectra. Mr. G. Gubellini and Mr. V. Cacciari are gratefully acknowledged for technical assistance. This work was supported by the Minister0 della Pubblica Istruzione and Consiglio Nazionale delle Ricerche. Registry No. [EuC2.2.1]'+, 65013-29-8; [ E U C ~ . ~ . I ] ~ +110316-F, 48-8; [ E ~ C 2 . 2 . 1 ] ~ + - 2 F -110316-49-9; , [TbC2.2.1I3+, 71238-22-7; [ Tb C 2.2.1] '+-F-, 1 10316-50-2; [ TbC 2.2.1 ] '+-2F-, 1 10353-5 1 -0; fluoride, 16984-48-8.
Raman Spectroscopic Study of the Molecular Conformation of a-n-Alkyl-w-hydroxyoligo(oxyethy1ene) Surfactants in the Solid State Hiroatsu Matsuura* and Koichi Fukuhara Department of Chemistry, Faculty of Science, Hiroshima University, Higashisenda-machi, Naka- ku, Hiroshima 730, Japan (Received: April 9, 1987) Raman spectra of 23 homogeneous nonionic surfactants, CH3(CH2),,(OCH2CH2),0H (C,E,), where n = 6, 8, 10, 12, and 16 and m = 1-8, have been measured in the solid state at liquid nitrogen temperature. The molecular conformation of these surfactants has been examined on the basis of the spectral analysis by utilizing the accordion vibration, conformation-spectrum correlations, and normal coordinate treatment. Molecules of the C,E1 compounds have been shown to take two conformationalforms in the crystal; while the alkyl group is in an extended conformation in common, the -OCH2CH20H group is in the gauche conformation in one form and it is in the trans conformation in the other. For the C,E3 compounds (their solid being obtained by rapid cooling) and the C,E2 compounds, the molecule takes an extended conformation in both the alkyl and oxyethylene groups with a gauche -OCH2CH20H terminal group. In molecules of the C,E3 surfactants (their solid being obtained by slow cooling) and the C,E, surfactants with m I4, the oxyethylene chain is in a helical conformation similar to the 72 helix of poly(oxyethy1ene) and the alkyl chain is in an extended conformation except for the gauche -CH2CH2CH20- part at the joining section to the oxyethylene chain. The molecular conformation of the C,E, surfactants in the solid state is shown to change from the highly extended form to the helix dominative fxm, as the number of oxyethylene units ( m ) is increased. This conformational transition takes place around m = 3, which corresponds approximately to one turn of the 7, helix for poly(oxyethy1ene). The conformational behavior of homologous C,E, surfactants may be elucidated by the conformational competition between the oxyethylene chain, which intrinsically favors the helical structure, and the alkyl chain, which favors the extended structure. The molecular conformation of the CUE, materials with m 2 4 yields tilting of the alkyl chain away from the oxyethylene helix axis, consistent with the previous results of X-ray diffraction studies. Introduction cu-n-Alkyl-w-hydroxyoligo(oxyethylene)s,CH3(CH2),-,. (OCH,CH,),OH (C,E,), constitute a typical family of nonionic surfactants, and their physical properties in solution are closely related to morphological aspects of their aggregates. Confor0022-3654/87/2091-6139.$01.50/0
mational behavior of the surfactant molecules plays an important role in determining the shape and size of the A (1) Mittal, K. L.; Lindman, B., Eds. Surfacfants in Solution; Plenum: New York, 1984; Vol. 1-3.
0 1987 American Chemical Society
6140 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
study of the molecular conformation in the solid state is basically important from a viewpoint that the solid-state conformation should be strongly correlated to the molecular forms of lowest energy in solution. Polyethylene (-CH2-), and poly(oxyethy1ene) (-OCHzCH2-), are relevant model compounds of the hydrophobic alkyl and hydrophilic oxyethylene moieties, respectively, of the C,E, surfactants. The solid-state conformation of these polymers has been well established owing to the extensive investigation^.^ The molecular conformation of polyethylene is all-trans; Le., all of the carbon atoms constituting the molecular chain are coplanar! For poly(oxyethylene), on the other hand, the conformation about the CHzCH2-OCHz bond is trans and the conformation about the OCH2-CH20 bond is gauche; Le., this polymer takes a helical structure with the consecutive trans-gauche-trans conformation about the O-CH2-CH2-0 bond axes.5 The conformation of the nonionic surfactants C,E, in the solid state has been studied by Rosch.6 According to his interpretation of the observed X-ray diffraction data of polydisperse materials, the all-trans zigzag conformation of the oxyethylene chain is transformed into the “meander” conformation with an increase of average chain length. The proposed meander model bears, however, heavy steric hindrance and is unacceptable in view of the rotational isomeric state.7 Raman spectroscopic studies on the conformation of nonionic surfactants have been reported by Kalyanasundaram and Thomass and Cooney et aL9 The materials they treated were polydisperse preparations with heterogeneous chain length distribution. Since their spectral analyses were based primarily on empirical band assignments, conformational interpretations they gave for their competent spectral data were insufficient or irrelevant. On the other hand, Dorset’O has studied the crystal packing of homogeneous nonionic surfactants CsE, (.m = 1-5) and C12E, ( m = 1-9) by electron and X-ray diffraction methods and presented the helical conformation of the oxyethylene chain linked with the tilted alkyl chain for m 2 4. In our previous Raman spectroscopic studies, we have reported the molecular conformation of ClzE, ( m = 3-8) in the solid state” and the conformational transition in a series of C8E, and CIzEm.l2 These studies have revealed that there are several types of molecular conformation in the solid state and that, as the number of oxyethylene units ( m ) is increased, the conformation changes from a highly extended form to a helix dominative form. These results on the conformational behavior of C,E, are stimulative enough to undertake a thorough investigation of the conformational morphology of a family of these nonionic surfactants. In the present work, we have investigated in detail the molecular conformation of 23 homogeneous C,E, surfactants with n = 6, 8, 10, 12, and 16 and m = 1-8 in the solid state by Raman spectroscopy, in conjunction with normal coordinate calculations. In the analyses of the spectra, spectroscopic data for related compounds, in particular a series of 1-alkanols CH3(CHz),10H (C,E,), are of great use.
Experimental Section Nonionic surfactants treated in this work are C6E, ( m = 1 and 2), C8E, ( m = 1-4), CloE, ( m = 1-4,6, and 8), C,,E, ( m = (2) Degiorgio, V.; Corti, M., Eds. Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; North-Holland: Amsterdam, 1985. (3) Tadokoro, H. Structure of Crystalline Polymers; Wiley: New York, 1979. (4) Bunn, C. W. Trans. Faraday Soc. 1939, 35, 482-91. ( 5 ) Takahashi, Y.; Tadokoro, H. Macromolecules 1973, 6, 672-5. (6) Rbch, M. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1967; pp 753-93. (7) Flory, P. J. Statistical Mechanics of Chain Molecules; Wiley: New York, 1969. ( 8 ) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1976, 80, 1462-73. (9) Cooney, R. P.; Barraclough, C. G.; Healy, T. W. J . Phys. Chem. 1983, 87, 1868-73. (IO) Dorset, D. L. J . Colloid Interface Sei. 1983, 96, 172-81. ( I 1) Matsuura, H.; Fukuhara, K. Chem. Lett. 1984, 933-6. (12) Matsuura, H.; Fukuhara, K. J. Phys. Chem. 1986, 90, 3057-9. (13) Wrigley, A. N.; Stirton, A. J.; Howard, E., Jr. J . Org. Chem. 1960, 25, 439-44.
Matsuura and Fukuhara 1-8), and C&m (m = 4,6, and 8). Most of the materials were supplied by three manufacturers: C6E1and C6E2by Tokyo Kasei Kogyo Co., CsE3and CsE4by Sigma Chemical Co., and CIoE, ( m = 3, 4, 6, and 8), CI2Em,and C&, by Nikko Chemicals Co. The materials of CsE1,C&, CloE1,and CIOE2were prepared in our laboratory from pertinent 1-chloroalkanes and mono- or diethylene glycol monosodium salt.I3 CIo&, C12E,, C12E*,and C16Em ( m = 4,6, and 8) are solid at room temperature, but other lower members are liquid. Several 1-alkanols (C,b), supplied by Tokyo Kasei Kogyo Co., were also included in the present work for the purpose of investigating the spectral and conformational relationship between C,E, and C,Eo. The materials employed were purified by vacuum distillation. All of the Raman spectra for the C,E, surfactants and l-alkanols were measured in the solid state at liquid nitrogen temperature. The materials, which were solid at room temperature, were once melted before the spectral measurement. We have carefully examined dependence of the solidification conditions on the spectra, by changing cooling rate or by annealing the solidified substance. We have in fact found that rapid cooling with less than 3 min to reach near liquid nitrogen temperature and slow cooling with more than 30 min to crystallize the substance completely gave quite different spectra for C8E3and CI2E3. The Raman spectra were recorded on a JEOL JRS-400D spectrophotometer equipped with a Hamamatsu R649 photomultiplier, by using a 514.5-nm excitation line of an NEC GLG3200 argon ion laser. A band-pass filter was used to eliminate the laser plasma emission. For spectral calibration, the neon emission spectrum was employed.
Normal Coordinate Treatment Normal coordinate treatment is a powerful tool for interpreting observed Raman and infrared spectra accurately, because it gives not only wavenumbers of the normal vibrations responsible for the observed bands but also modes of the vibrations. In order to obtain such vibrational information, we need the force field of the molecule in question. The force field is desired to be as precise as possible so that the results obtained are authentic enough to lead to unequivocal interpretation of the spectra. In our previous studies,I4J5we have derived the force field of an oxyethylene chain from the observed Raman and infrared data for the model compounds of poly(oxyethylene), namely R(OCH2CH2),0R’ ( R , R’ = C H 3 or H) and CD3(OCDzCDz),OCD, with m up to 3. The force field of the alkyl group has been well determined by a systematic normal coordinate analysis.16 Accordingly, we used these force fields, expressed in terms of the group coordinates,” of the oxyethylene and alkyl groups for the present normal coordinate treatment of the C,E, molecules. The computation was carried out with a program MVIB” which was developed for treating normal coordinates of chain molecules such as C,E, molecules. With this program, users need indicate only minimal input of atomic groupings and conformation of the molecule, since the pertinent force constants, structural parameters, and other data necessary for the calculation are provided in advance in computer files. Thus, the calculation was straightforward even for large C,E, molecules with numerous possible conformations. This program is currently capable of treating chain molecules consisting of 120 or fewer atoms. Results and Discussion ( a ) Basis for the Conformation Determination. The Raman spectra of the C,E, surfactants have been analyzed as precisely as possible in order to determine the molecular conformation. Our conformation determination is based on the accordion vibration (14) Matsuura, H.; Fukuhara, K.; Tamaoki, H. J. Sei. Hiroshima Uniu., Ser. A 1985. 49, 89-1 13. (15) Matsuura, H.; Fukuhara, K.; Tamaoki, H. J . Mol. Struct. 1987, 156, 293-301. (16) Shimanouchi, T.; Matsuura, H.; Ogawa, Y.; Harada, I. J . Phys. Chem. Ref Data 1978, 7, 1323-443. (17) Matsuura, H.; Tasumi, M. Vib. Spectra Struct. 1983, 12, 69-143.
a-n-Alkyl-w-hydroxyoligo(oxyethy1ene)Surfactants
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6141
I
8
,
1600
1400
,
1200
l
1000
I
l
000
I
600
l
I
400
~
I
l
200
Wavenumber/cm-l Figure 2. Raman spectra of C6Ez,C8E2,CloE2,and C12E2(type B) in the solid state.
1600
1400
1200
1000
000
600
Wavenumber/cm-l Figure 1. Raman spectra of C6E1and C8El(type A), CloEl.and ClzEl (type A'), 1-tridecanol(C13Eo),and 1-pentadecanol(CISEL)) in the solid state.
to estimate an overall conformational state of the molecule, conformationspectrum correlations of the oxyethylene chain to determine local conformations, and normal coordinate treatment to ascertain the detailed conformation. ( i ) LAM-1 Accordion Vibration. For estimating an overall conformation of the molecule, the low-wavenumber vibrations associated with deformations of the molecular skeleton are useful, because their wavenumbers are highly sensitive to the molecular conformation. In particular, the accordion vibration of the extended planar zigzag structure is important for deducing an extent of the extended part of the molecule, since the wavenumber of this vibration is inversely proportional, to a good approximation, to the length of the extended chain.18-19 The accordion vibration is associated with the fundamental longitudinal acoustic mode (LAM-1) of the chain skeleton. The LAM-1 mode gives rise to a strong Raman band because of its large polarization variation with the vibration. For n-alkane molecules, the LAM-1 band is observed in a low-wavenumber region, Le., below 300 cm-I for octane and longer homologues. The prominent feature of the LAM-1 Raman band and the established LAM-1 wavenumbers available for a series of n-alkane molecules2b22 are of great use for evaluating the length of the extended structure of the C,E, molecules. The wavenumber of Mizushima, S.;Shimanouchi, T. J. Am. Chem. Soc. 1949,71, 1320-4. Schaufele, R. F.; Shimanouchi, T. J. Chem. Phys. 1%7,47,3605-10. Schaufele, R. F. J . Chem. Phys. 1968, 49, 4168-75. Olf, H. G.; Fanconi, B. J . Chem. Phys. 1973, 59, 534-44. (22) Takeuchi, H.; Shimanouchi, T.; Tasumi, M.; Vergoten, G.; Fleury, G . Chem. Phys. L e f t . 1974, 28, 449-53. (18) (19) (20) (21)
the LAM-1 accordion mode is, however, affected by interlamellar interactions and terminal groups attached to the chain skeleton.2f26 In order to examine this effect, we treated in the present work several 1-alkanol molecules as well, whose structures have been determined by an X-ray diffraction method. ( i i ) Conformation-Spectrum Correlations. Correlations between the conformation and vibrational spectra of the oxyethylene Making use of chain have been derived in our previous such correlations, the bands that are characteristic of particular local conformations involved in the C,E, molecule are readily identified in the spectra. These conformation-spectrum correlations have been in fact applied to a conformational analysis of poly(oxyethy1ene) in the molten state and in solution.27 (iii) Normal Coordinate Treatment. Treatment of normal coordinates is one of the sophisticated methods for analyzing Raman and infrared spectra. In spite of considerable size of the molecules to be treated (e.g., 107 atoms for Cl6E8)and many possible conformers to be considered, the calculations are almost straightforward owing to the intelligent computer program.17 The calculated results include various important quantities such as vibrational wavenumbers, potential energy distributions, and displacements of atoms during the vibration. These data are very useful for a detailed analysis of the C,E, spectra to establish the molecular conformation. ( b ) Classifcation of Spectral Types. The Raman spectra of C,E, in the solid state at liquid nitrogen temperature are shown in Figures 1-7. While most of the C,E, surfactants gave single spectral features irrespective of the solidification conditions, C8E3 and C12E3gave two types of spectra exhibiting distinguishing features; one spectrum was obtained by the rapid cooling of the liquid and the other by the slow cooling. The Raman spectra of Hsu, S. L.; Krimm, S. J . Appl. Phys. 1977, 48, 4013-18. Minoni, G.; Zerbi, G. J . Phys. Chem. 1982, 86, 4791-8. Minoni, G.; Zerbi, G.; Rabolt, J. F. J . Chem. Phys. 1984,81,4782-9. Viras, K.; Teo, H. H.; Marshall, A,; Domszy, R. C.; King, T. A.; Booth, C. J . Polym. Sci., Polym. Phys. Ed. 1983, 21, 919-21. (27) Matsuura, H.; Fukuhara, K. J . Polym. Sci., Part E 1986, 24, (23) (24) (25) (26)
1383-400.
~
~
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
6142
1
Matsuura and Fukuhara
&E3 (rapid cooling)
-
I
/
/
1600
1
l
1400
,
/
I200
,
/
,
/
800
1000
/
l
600
~
l
1
1
200
400
l
0
Wavenumber /cm-' Figure 3. Raman spectra of CBE3(rapid cooling), CloE3,and CI2E3 (rapid cooling) (type B) in the solid state.
~!
1600
1400
1200
800
1000
600
400
200
0
Wavenumber /cm-' Figure 5. Raman spectra of CIoE4,CloE,, and CIoE8(type C) in the solid
state.
C8E3 (slow cooling)
'~
i
i I -
1600
L
,
1
1400
,
1
1200
,
1
1000
,
1
000
1
,
600
,
I
400
,
200
I
0
Wavenumber/cm-' Figure 4. Raman spectra of C8E3(slow cooling) and C8E4(type C) in
the solid state. C,E, may be classified into four types, namely, types A, A', B, and C, on the basis of their characteristic spectral features. Type A is characterized by (1) two prominent bands assignable to the LAM-1 vibrations in the region below 300 cm-I, (2)a group of three bands in the region 860-900 cm-I, (3) an appreciable band at about 1165 cm-', and (4) a well-defined band at about 1415 cm-'. The spectra of C6E1and C8El belong to this type (Figure 1). Type A' is similar to type A, but exhibits apparently one LAM-1 band. The spectra of CIoEIand CI2E1belong to this type (Figure 1
1).
Type B is characterized by (1) a prominent LAM-1 band, (2) a weak band at about 690 cm-I, (3) a pair of bands at about 860 and 890 cm-', and (4) strong band(s) at about 1500 cm-I. The spectra of C6E2, C&, C8E, (rapid cooling), CI0E2,C10E3,CI2E2, and Ci2E3 (rapid cooling) are of this type (Figures 2 and 3). Type C is identified by (1) rather indistinct LAM-I bands, (2) a band at 285-310 cm-I, (3) a band progression in the region 500-600 cm-', (4)a rather complex spectral pattern in the region 800-900 cm-', (5) a weak band at 935-940 cm-', (6) a welldefined band at 1230-1235 cm-', and (7) a strong band at about 1490 cm-'. The spectra of C8E3(slow cooling), C8E4,CloE4,C10E6, C I O E ~C12E3 , (slow cooling), CI2E4,C&,, CI2E7,CI2E8, C16E4, C16E6, and Cl6E8 belong to this type (Figures 4-7). It should be noted, however, that the conformation of CI2E7is
I
I
1600
-I
1400
1200
t
,
1000
~
~
000
~
600
~
400
1
200
Wavenumber /cm-' Figure 6. Raman spectra of CI2E3(slow cooling), C12E4,C,,E5, CI2E6. C,,E,, and C12Es(type C) in the solid state.
different from the others that belong to this type; details will be given later. ( c ) Determination of the Molecular Conformation. ( i ) Type A . The Raman spectra of C6E, and CsEl (type A) and of CloEl
'
"
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6143
a-n-Alkyl-w-hydroxyoligo(oxyethy1ene)Surfactants TABLE I: LAM-1 Wavenumbers for C,E,, 1-Alkanols, and n-Alkanes
C,El compdb
NO
'vc
C,E2 and C,E3 compdb 'vc
9 10 11 12 13 14
1-alkanol compdb
n-alkane 7
compd
;c,d
C9H20
249 230 206, 219 195 183 169
CIOH22
CllH24
205 175
15 16 17 18 19
155 152 140 136
20 21
126
C12H26
C13E0 (I) C13E0 (I1) C14E0 C14E0 C15E0
(I) (I1) (I)
C15E0
(I1)
184 178 173 168 163 157
C13H28 C14H30
158 C16H34
C17H36 C18H38 C19H40
C20H42 CZ1H44
150 141 133 126 122 115
"The number of backbone atoms constituting the planar structure. bThe molecular form is indicated in parentheses. 'LAM-1 wavenumber, in reciprocal centimeters, of the extended part of the molecule. dReference 21. tively. The molecular form I gives a higher wavenumber LAM-1 band, and the form I1 gives a lower wavenumber LAM-1 band. In conformity with this spectral interpretation for the 1-alkanols, the higher wavenumber LAM-1 band of C6El and CsEl is assigned to the molecular form
C16E4
T T
C-C-C-
...
T T T T G
-C-C-O-C-C-OH
I1I
and the lower wavenumber LAM- 1 band is assigned to the molecular form T T
T T T T T
C-C-C- ...-C-C-O-C-C-OH
IV
l
1600
:
I
1400
1
l
1
l
1200
1000
600
000
400
200
0
Wavenumber/cm-1 Figure 7. Raman spectra of CI6E4,C16E6, and C16EB (type c )in the solid
state. and C12EI(type A') are shown in Figure 1, together with those of 1-tridecanol C13EO and 1-pentadecanol CI5EO,which have the same number of backbone atoms as CloEland C12EI,respectively. The observation of two LAM-1 bands for the type A compounds suggests that in crystal the molecules take two forms with different lengths of the planar skeleton. The solid-state spectra of C13EO and C15Eo,whose crystal structure at low temperature has been e s t a b l i ~ h e d , exhibit ~ ~ - ~ ~in fact two LAM-1 bands which are due to two molecular forms:
I
and T
T
C-C-C-C-
T
. .T
T
T
C-C-C-OH
I1
where T and G denote trans and gauche conformations, respec(28) Tanaka, K.; Seto, T.; Hayashida, T. Bull. Inst. Chem. Res. Kyoto Uniu. 1957, 35, 123-39. (29) Tanaka, K.; Seto, T.; Watanabe, A.; Hayashida, T. Bull. Inst. Chem. Res. Kyoto Wniu. 1959, 37, 281-93. (30) Seto, T. Mem. Coll. Sci. Univ. Kyoto, Ser. A 1962, 30, 89-107.
These assignments to the particular conformations, I11 and IV, are evidenced by the LAM-1 wavenumbers with reference to those for the corresponding n-alkane molecules with a fully extended conformation. The molecular forms of C6E1(111), C6E1 (IV), C& (111), and CsEl (IV) contain the planar structure consisting of 9, 10, 11, and 12 backbone atoms, respectively. The LAM-1 wavenumbers for C6E, (111), C6E1 (IV), C& (111), and C& (IV) are 255, 239, 221, and 203 cm-I, respectively, which are in agreement with 249 cm-' for C9H20,230 cm-' for CloH22,206 and 219 cm-' for ClIHz4,and 195 cm-' for CI2H26, respectively.21 The LAM-1 wavenumbers for the relevant compounds are summarized in Table I. It should be remarked here that stronger interlamellar forces in the hydroxyl-ended chain than in the methyl-ended chain make the LAM-1 wavenumbers for C,E, and C,Eo higher than those for the corresponding n-alkanes with similar planar structure. The normal coordinate treatment on C6El (III), C6E1 (IV), CsEl (111), and C& (IV) gave the LAM-1 wavenumbers 236, 225, 202, and 185 cm-l, respectively, which are compared with the observed wavenumbers 255, 239, 221, and 203 cm-'. The observed wavenumbers are in general higher than the wavenumbers calculated for an isolated single molecule. This wavenumber difference is ascribable to intermolecular interactions in the crystal, which increases the effective LAM- 1 wavenumbers. A characteristic spectral feature consisting of three bands is noted for type A in the region 860-900 cm-'. The normal coordinate calculation indicated that the band at about 860 cm-I is assigned to the molecular conformation I11 and is associated with the CH2 rocking mode as coupled with the C-OH stretching of the -0-C1H2-CIIH2-OH group in the trans(0-CI)-gauche(CI-CII) conformation. On the other hand, the strong band at about 890 cm-I is assigned to both of the conformations I11 and IV and is due to the CH, rocking and CI-CIl stretching modes of the methyl-terminal CH3-CIH2-CI1H2- group in the trans(CI-CII) conformation. The conformation IV is further evidenced by an observation of a band at about 9 8 0 cm-', which is assigned, according to the normal coordinate calculation, to the symmetric
6144
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Matsuura and Fukuhara
for CI3EOand CI5EO,respectively, each of which exhibits two CII-O-CIII stretching mode, as coupled with the alkyl C-C stretching, of the -CIH~-C~~H~-O-CI~~H~-CI~H~-OH part with LAM-1 bands associated with the molecular forms I and 11. the trans(CI-CII)-trans(CII-O)-trans(O-CIII)-trans(CIII-CIV) Accordingly, the wavenumbers of the prominent LAM- 1 bands conformation. A band at about 1165 cm-I is also significant for for CIoEland C12Elcorrespond to the conformation IV, or the yT form, which contains 14 and 16 coplanar backbone atoms, the type A compounds. The normal coordinate analysis indicates respectively (see Table I). that this band is characteristic of the -CIH2-CIIH2-CIIIH2-OThe above discussions suffice to conclude that the conformastructure in the trans(CI-CII)-trans(CII-CIII)-trans(CIIl-O) tional state of the molecules of type A' (CloEland Cl2EI)is the conformation as found in the molecular forms I11 and IV and that same as that of type A (C6E, and CsEl); the molecules in the it is associated with the rocking mode of the CH2 groups involved crystal take two conformational forms, the y form and the yT form. in this structure. The observed bands in the region 300-600 cm-' ( i i i ) Type B. The Raman spectra of C,E2 (n = 6, 8, 10, and are also consistent with the coexistence of the molecular forms 12), C8E3(rapid cooling), CI0E3,and CI2E3(rapid cooling) are I11 and IV. of this type (Figures 2 and 3). Each of these spectra shows a The examinations of the spectra of C6El and CsEl as described prominent LAM- 1 band characteristic of the extended chain above have established that molecules of the type A compounds structure. For the C,E2 series, the LAM-1 wavenumbers are 205, take two different forms, 111and IV, in the crystal. We shall call 175, 155, and 140 cm-' for n = 6, 8, 10, and 12, respectively, and the conformation I11 the y form and the conformation IV the yT for the C,E3 series, they are 152, 136, and 126 cm-' for n = 8, form, where the subscript T refers to the t r a n s ( C 4 ) conformation 10, and 12, respectively. These wavenumber values imply that, in the terminal -OCH2-CH20H group. Definition of the y form on the increase of the alkyl chain length or of the oxyethylene will be more relevant when we discuss the molecular conformation chain length, the extent of the extended structure in the molecule of the type B compounds. increases. This finding suggests that both the alkyl and oxyFor all of the compounds of types A and A' studied, a prominent ethylene groups are involved, at least in part, in this extended Raman band is noted at about 1415 cm-I, which is never observed structure. for types B and C. The observation of this band bears a close The LAM-1 wavenumbers for the C,E2 compounds are conresemblance to that made for polyethylene and n-alkanes with siderably lower than those for the yT form of the corresponding orthorhombic and monoclinic unit cells.31 These compounds, C,El compounds with the same n value. The wavenumbers for unlike triclinic or hexagonal n-alkanes, exhibit a Raman band at C6E2,CsE2, and CloE2are actually nearly coincident with those about 1415 cm-I, which, along with a band at about 1445 cm-', for the yT form of C8El (203 cm-I), CloEl (176 cm-I), and C12E, has been interpreted to be a crystal-field split component of the (1 59 cm-I), respectively (see Table I). These results, together CH2 scissoring mode.31 This interpretation encourages us to assign with other spectral evidence to be mentioned later, lead to the the band at about 1415 cm-I for types A and A' of C,E, to a molecular form for C,E2 of type B: component split by the crystal field, the other component being identified at about 1445 cm-' as a stronger band. It follows that T T T T T T T T T G C-C-C-. - C - C - 0 - C- C- O-C-C-OH the crystal structure of the compounds of types A and A' is similar to that of orthorhombic or monoclinic n-alkanes. The crystal-field v splitting of the CHz scissoring mode is also observed for longer which has 12, 14, 16, and 18 coplanar backbone atoms for C6E2, 1-alkanols (e.g., CI2EOthrough CzoEo)at liquid nitrogen temCsEz, ClOE2, and CI2E2, respectively. The LAM- 1 wavenumbers perature, but not for shorter homologues (e.g., C8Eo through for these C,Ez compounds, 205, 175, 155, and 140 cm-I, are in CIIEo)at the same temperature. accord with those for ClzHZ, (195 cm-I), CI4H3, (169 cm-I), (ii) Type A'. Overall spectral characteristics of the type A' C16H34(150 cm-I), and C1gH38 (133 cm-1).21 The normal cocompounds (CloEIand C12E,)are essentially the same as those ordinate calculations on the conformation V of C6E2, C&, CIOEz, of the type A compounds (C6E1 and CsEl) except for the oband C12Ezgave the LAM-1 wavenumbers 194, 170, 150, and 132 servation of an apparent single LAM-1 band. Namely, the cm-', respectively, in agreement with the observed. compounds of type A' show, in addition to the LAM- 1 band, a For the C,E3 compounds of type B, on the other hand, the group of three bands in the region 860-900 cm-', bands at about observed LAM-1 wavenumbers are 152, 136, and 126 cm-' for 980 and 1165 cm-', and a pair of bands at about 1415 and 1445 n = 8, 10, and 12, respectively. These wavenumbers, when cm-'. These features, as have been interpreted for the type A compared with those for the pertinent n-alkanes (see Table I), compounds, are highly suggestive of the same molecular structure lead to the molecular form in the crystal as found for the type A compounds. This indicates T T T T T T T T T T T T G that the molecules of CloEl and C12Elmay assume two conforC-C-C- C- C- 0 - C- C-0- C-C- 0-C-C- OH y and yr forms. mations of the VI The apparent single LAM-1 band for the type A' compounds is interpreted, on the basis of the following considerations, to be which has 17 coplanar backbone atoms for CsE3, 19 for CloE3, due to overlapping of two LAM-I bands for the y and yT forms. and 21 for C12E3. Figure 1 shows that the intensity of the higher wavenumber The molecular forms V for C,E2 and VI for C,E3 will now be LAM-1 band decreases in going from C6El to C8El. If the further discussed by examining local conformations of the molstructure of CloEIand Cl2E1is of the same type as C6E1and CsEl, ecule. The type B compounds show a band around 970-980 cm-', a further decrease in intensity of the higher band is expected. which is worse defined for the longer homologues. This band is Close inspection of the spectrum of CloEl shows in fact that a part characteristic of the R-CIH2-CIIHz-O-CIIIH2-ClvH2-Odistinct shoulder is noted at 183 cm-' on the higher wavenumber with the trans(CI-CII)-trans(CII-O)-trans(O-CIII)-transside of the prominent LAM-I band at 176 cm-'. The real existence (CIII-CIV) conformation, similar to the band at about 980 cm-I of this shoulder has been confirmed by measuring Raman spectra for types A and A'. The band at about 860 cm-I for type B is of this compound several times. The intensity decrease of the associated primarily with the -O-CIHz-CIIH,-OH group in the higher wavenumber band is further manifested in the spectrum trans(0-CI)-gauche(CI-CI1) conformation, in accordance with of C I 2 E Iwhich , shows seemingly one sharp LAM-1 band at 159 the corresponding band for types A and A'. The type B comcm-'. This intensity behavior of the LAM-I band with increasing pounds do not show, unlike the type C compounds, a distinct band alkyl chain length must be due to intermolecular interactions at 1230-1235 cm-', which is due to the interior -0between the molecules of the y and yT forms in the crystal. It CIH2-CIIH2-0- group in the trans(O-CI)-gauche(Cl-CII)should be mentioned that the wavenumbers of the observed trans(CII-0) conformation, but show in this region only a weak LAM-I band for CloEl(176 cm-l) and ClzEl(159 cm-I) arevery band due to the terminal -0-CIHZ-CIIH2-OH group in the close to 178 and 157 cm-' for the lower wavenumber LAM-1 band trans(O-CI)-gauche(C1-CI1) conformation. Also the compounds of type B do not give a band at 935-940 cm-' characteristic of the type C compounds having the -O-CIH2-CIIH2-O(31) Boerio, F. J.; Koenig, J. L. J. Chem. Phys. 1970, 52, 3425-31. n
a-n-Alkyl-whydroxyoligo(oxyethy1ene) Surfactants
CIIIHZ-CIVHZ-O-part with the gauche(CrCll)-trans(CIl-O)-
trans(O-CIII)-gauche(CIII-Clv) conformation. These spectral findings further support the molecular forms V and VI for the type B compounds. One of the other distinguishing features of type B is strong band(s) at about 1500 cm-I; the other types do not show a band a t so high a wavenumber as this. This band is assigned to the CH2 scissoring mode of the interior -0-CH2-CH2-0- group in the trans(C-C) conformation, in the light of the spectral observation that solid 18-crown-6, which contains in the molecule the same conformational segment as above, also exhibits a strong band at about 1500 ~ m - ' . ) ~Another distinction of type B from the other types is an observation of a weak band at about 690 cm-I. This band is reasonably assigned to the C-OH torsional mode (or, in other words, C-0-H out-of-plane bending mode) of the terminal -0-CH2-CH2-OH group. The discussions given above have established the molecular form of the type B compounds. The conformations V and VI thus determined are called the y form. The conformation I11 for the C,E1 compounds is a special case of the conformation V or VI in that no interior -0-CH2-CH2-0group is involved in 111. (iu) Type C. The Raman spectra of C8E3 (slow cooling), CsE4, CloE, ( m = 4, 6, and 8), CI2E3(slow cooling), CI2Em( m = 4-8), and CI6Em( m = 4, 6, and 8) belong to this type (Figures 4-7). For CsE3and CI2E3,we obtained the Raman spectra of types B and C by cooling the liquid substance rapidly and slowly, respectively. The spectral features of the two types are quite different from each other. While the type B spectra exhibit a prominent LAM-1 band, the type C spectra show much less prominent bands assignable to split components of the LAM- 1 mode (to be discussed later) in the wavenumber region considerably higher than that for type B. This observation is indicative of much shorter extended structure involved in the molecule of type C than type B. Examination of the type C spectra shows that the compounds having the same number of alkyl carbons ( n ) give substantially the same spectral pattern in the region below 250 cm-': bands are observed at 220-250 cm-I for C,E,, 210-215 cm-' for CloE,, 150-200 cm-' for CI2Em(except for m = 7), and 120-150 cm-' for CI6Em.These features of the spectra indicate that the C,E, molecules of type C with the same n value have the extended planar structure of the same length. Investigation of the type C spectra also points out that the feature in the region 500-600 cm-I is quite similar among the compounds having the same number of oxyethylene units ( m ) . The band progression in this region is associated with the V I 6 optical branch of poly(o~yethy1ene)~~ with the 72 helical structure (seven units of OCHzCHzand two helical turns per fiber period) having the trans-gauche-trans conformation about the O-CH2-CH2-0 bonds. The number of progression bands increases with an increase of OCH2CH2units incorporated into the helical structure. For the C,E, compounds of type C we studied, the number of the observed bands in this region is four for C,E3 (n = 8 and 12), five for C,E4 ( n = 8, 10, 12, and 16), six for CI2E5,and so on. This observation indicates that, as the number of oxyethylene units ( m ) in the C,E, molecule increases, an extent of the helical structure of the oxyethylene chain increases. The helical structure of the oxyethylene chain is also evidenced by an observation of a band at 285-310 cm-', which is associated with the v I 7 branch of poIy(o~yethy1ene)~~ and converges to 281 cm-I for the polymer.34 The type C compounds show a well-defined characteristic band at 1230-1235 cm-I. This band, corresponding to the 1233-cm-I band of solid poly(o~yethylene),~~ is due to the antisymmetric CH2 twisting mode of the interior -0-CIH2-CIIH2-0group in the trans(O-C1)-gauche(CI-Cll)-trans(CII-O) c ~ n f o r m a t i o n .It~ ~ is thus confirmed that at least one of the interior -0-CH2CH2-0- groups in the molecule takes the trans-gauche-trans conformation. In addition to the band at 1230-1235 cm-', the (32) Matsuura, H.; Fukuhara, K., unpublished work. (33) Matsuura, H.; Miyazawa, T. Bull. Chem. Soc. Jpn. 1969,42, 372-8. ( 3 4 ) Koenig, J. L.; Angood, A. C . J . Polym. Sci., Part A-2 1970, 8, 1787-96. (35) Matsuura, H.; Fukuhara, K. J . Mol. Struct. 1985, 126, 251-60.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6145 type C compounds give a band at 935-940 cm-I, which none of the compounds of types A, A', and B give. This band is assigned to the C H 2 rocking mode of the -O-CIH2-CIIH2-OCIIIHZ-CIVHZ-O-part with the gauche(C1-CII)-trans(CII-O)trans(O-CIII)-gauche(CIII-CIV) c o n f o r m a t i ~ n Examination .~~ of the spectra also indicates that the compounds of type C do not show a band characteristic of the -O-CH2-CH2-O- group in the trans(C-C) conformation, as observed for the type B compounds at about 1500 cm-I. The accumulated spectral evidence given above now leads to the conformation of the oxyethylene chain in the C,E, molecules of type C: T G T T
-C-C-C-O-C-C-O-
T
T O T
T Q
..-O-C-C-O-C-C-OH
VI1
which has the same helical structure as solid poly(oxyethy1ene). In order to determine the conformation of the alkyl group, we now return to the consideration of the LAM-1 bands. The relationship between the LAM-1 wavenumber and the length of the extended structure, as given in Table I, is not applicable to the molecules of type C, since the end effect of the extended chain on the LAM-1 mode must be significant owing to the nonextended helical structure of the adjoining oxyethylene moiety. Vibrational coupling between the LAM-1 mode of the extended part and other skeletal deformation modes is also expected to be large for these molecules, as suggested by the diminished Raman intensities of the LAM-1 bands in comparison with those for types A, A', and B. Accordingly, we utilized normal coordinate treatment to determine the conformation of the alkyl group. The calculation of normal coordinates indicated that the observed Raman bands in the region below 250 cm-I for the compounds of type C, except for CI2E7,agree only with the molecular form:
VI11
The calculated wavenumbers for CsE3 are 222,226, and 248 cm-' in comparison with the observed wavenumbers 216, 226, and 251 cm-I, and the calculated wavenumbers for &E3 are 158, 173, 177, and 198 cm-' in comparison with the observed wavenu'mbers 150, 174, 183, and 201 cm-'. It is important to note that the -CIH2-CIIH2-CIIIH2-0group in VI11 adjacent to the oxyethylene group is in the trans(CI-CII)-gauche(Cll-CllI)-trans(CIII-O) conformation. The conformational sequence of VI11 is supported by an observation of a weak band at 800-810 cm-I which is due to the C H 2 rocking mode of the alkyl chain in this conformation. This band is, however, not necessarily distinct for some of the type C compounds. The conformation VI11 determined this way for the type C compounds except for CI2E7will be called the /3 form. Sequential comparison of the spectra from CI2E3(slow cooling) to C12E8in the region below 200 cm-I reveals that only C12E7 exhibits a spectral pattern largely different from mutually similar patterns for the other homologues (Figure 6): for CI2E7,the feature at 170-200 cm-I is missing and is likely to be shifted down to 130-165 cm-'. It is also noted that CiZE7 does not exhibit a band around 1045 cm-', while all of the other C12E, compounds of type C invariably show a band at this wavenumber. These findings suggest that the molecule of C12E7has a peculiar conformation unlike the others. The normal coordinate calculation demonstrated that the observed spectrum for this compound is best explained by the molecular form T
T
C-C-C-.
.
T T T T T G T T
-C-C-C-O-C-C-O-
T e..
T G T T Q - O-C-C-O-C-C-OH
IX
where the -CIH2-CIlHz-CIIIHz-O- group adjacent to the oxyethylene group is in the trans(CI-CII)-trans(CII-CllI)-trans(CIII-0) conformation (compare with the molecular form VIII). The molecular conformation IX is called the a form. (d) Discussion of the Results. (i) Conformational Forms of C,Jm in the Solid State. In the present work, we have presented the molecular conformation of the C,E, surfactants ( n = 6, 8,
6146
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
c x
Matsuura and Fukuhara TABLE 11: Conformational Forms of the C,E, Surfactants in the Solid State
m
5
n
10
2
Y+YT Y+YT
Y
3
4
5
6
7
8
rb%P P Y P P P Yb3@ P 0 P P P P P ‘y + yr implies that both the y and Y~ forms coexist in the crystal. Rapid cooling of the liquid. ‘Slow cooling of the liquid. 8
10 l2 16
4
( a1
(b) (C) (d) Figure 8. Skeletal conformation models of the C,E, molecule: (a) a form, (b) /3 form, (c) y form, and (d) yr form. The models of the C8E, molecule are shown as an illustration. ( 0 )Carbon atom (the terminal carbon represents a methyl group and others represent methylene groups); (0)oxygen atom (the terminal oxygen represents a hydroxyl group). The helix axis of the oxyethylene chain is indicated for the a and 9, forms.
10, 12, and 16; m = 1-8) in the solid state and indicated that there are several conformational forms, Le., the a, p, y, and YT forms. The skeletal models for these forms are constructed in accordance with the molecular conformations determined. These models are shown in Figure 8. The a form is conformationally a basic one, because it is an unmodified simple combination of an extended zigzag conformation of the entire alkyl chain and a helical conformation of the entire oxyethylene chain with the transgauche-trans conformation about the successive O-CH2-CH2-0 bonds. The hydroxyl-terminal part of the oxyethylene chain, -O-CH2-CH2-OH, is in the gauche(C-C) conformation for the LY form. The /3 form is slightly different from the a form; the -CIH2-CIIH2-CIIIH2-0part of the alkyl group at the joining section to the oxyethylene chain is in the trans(CI-CII)-gauche(CIl-C,lI)-trans(CII~) conformation and the remaining part of the molecule takes the same conformation as the a form. In other words, only the conformation about the CII-CIII bond as shown above is transformed from trans for the a form into gauche for the p form. It is thus understood that part of the alkyl chain adjoining to the oxyethylene chain is incorporated into the helical conformation of the oxyethylene chain (see Figure 8). The incorporated -CH2-CH2-CH2-(0) group is conformationally equivalent to an oxyethylene unit so that the helical part of the molecule for the p form is longer by approximately one oxyethylene unit than for the corresponding a form. The stable gauche conformation for the CH2CH2-CH20group of the p form is consistent with the experimental finding that CH30(CH,),0CH3 ( n = 3-5) molecules in the liquid state highly favor the gauche conformation about the C-C bond adjacent to the C-0 bond.36 A previous study3’ on the rotational isomerism of various alkyl ethers has also indicated that the relative stability of the gauche conformation to the trans conformation is much larger for the CC-CO bond than for the CC-CC bond. The y form is distinctively different from the basic a form. The oxyethylene chain assumes an extended zigzag conformation with the terminal -0-CIHZ-CIIH2-OH group in the gauche(CI-CII) conformation. The Y~ form as found for the C,EI compounds has the trans(Cl-CII) conformation for this terminal group. In contrast with the helix incorporation of the alkyl group in the /3 form, the oxyethylene chain is involved, together with the whole alkyl group, in the highly extended zigzag structure of the y form. (36) Matsuura, H.; Murata, H. J . Raman Spectrosc. 1982, J2, 144-8. (37) Shimanouchi, T.; Ogawa, Y . ;Ohta, M.; Matsuura, H.; Harada, I. Bull. Chem. Soc. Jpn. 1976, 49, 2999-3008.
’?
Y+YT Y+YT
The stability of the extended conformation of the oxyethylene chain is in accord with the fact that poly(oxyethy1ene) takes, in addition to the helical structure, all-trans extended structure in a less stable crystal m ~ d i f i c a t i o n . ~Also, ~ two of the 0-CH2CH2-0 groups in an uncomplexed 18-crown-6 molecule are in the trans-trans-trans conformation in the crystalline state.39 These data for the related compounds give support to our conformational model of the y form with the extended oxyethylene chain. It should be remarked that, for C,E, molecules, the y form and the LY form denote the same conformation. (ii) Chain Length Dependence of the Molecular Conformation. The conformational forms of the C,E, surfactants in the solid state are summarized in Table 11. Although all of the C,E, surfactants with n = 6-16 and m = 1-8 have not been completed, the present analysis has revealed some important conformational properties of the homologous C,E, molecules. Table I1 indicates that the mono(oxyethy1ene) molecules ( m = 1) all assume two conformations of the y and Y~ forms in the crystal, unlike the other homologues with m 2 2 which assume a single conformation. This distinctive property of the C,El compounds with the C-C-C-C-C-O-C-C-OH structure is analogous to a conformational property of 1-alkanols (C,Eo), C-C-C-C-C-C-C-C-OH, which also take two molecular conformations corresponding to the y and yr forms of C,E1.28-30 This suggests that the -CH2-0-CH2-CH2-OH group in the C,E, molecules conformationally behaves just like the -CH2-CH2-CH2-CH2-OH group in the C,Eo molecules. Table I1 shows that the C,E2 molecules and part of the C,E3 molecules (generally, in the case of the rapid cooling of the liquid) take the y form in the solid state. For these surfactants with m = 2 and 3, the oxyethylene chain is not long enough to make one turn of the 72 helical structure of poly(oxyethylene), for which 3.5 oxyethylene units form a turn.5 This consideration strongly suggests that four or more OCHzCH2 units are necessary for stabilizing the helical structure of the oxyethylene chain. In fact, the molecular conformation of C,E, with m 2 4 is the form consisting of the helical oxyethylene chain. It is remarked, however, that molecules of H(OCH2CH2),0H and CH3(OCH2CH2),0CH3 with m = 1-7 assume a helical conformation similar to the 72 h e l i ~ . ~ ~This , ~ ’is because, unlike the C,E, molecules presently treated, these molecules do not carry long n-alkyl chains. Accordingly, a more relevant statement should be made that, if an alkyl group, longer than at least pentyl, is linked to the oxyethylene chain, four or more oxyethylene units are necessary for forming the stable helical structure. On the other hand, if the oxyethylene part is not sufficiently long ( m 5 3), the conformation of this part is governed largely by the dominating alkyl group, and in consequence the oxyethylene group is joined with the alkyl group to form the extensive extended zigzag structure. The fact that some of the C,E3 surfactants take, depending on experimental conditions, two different molecular forms (the y and /3 forms) having the extended and helical oxyethylene ~~~~~~
~~~
~
~~
~~
(38) Takahashi, Y . ;Sumita, I.; Tadokoro, H. J . Polym. Sci., Polym. Phys. Ed. 1973, J J , 2113-22. (39) Dunitz, J. D.;Seiler, P.Acta Crystallogr., Sect. B 1974, 30, 2739-41. (40) Matsuura, H.; Miyazawa, T. Spectrochim. Acra, Purr A 1967, 23, 2433-47. (41) Matsuura, H.; Miyazawa, T.; Machida, K . Spectrochim. Acta, Part A 1973, 29, 771-9.
a-n-Alkyl-w-hydroxyoligo(oxyethy1ene)Surfactants chains, respectively, is explained by their specific oxyethylene chain length almost coincident with 3.5 oxyethylene units per turn for the 7, helix. The C,E, surfactants carrying the longer oxyethylene chain ( m Z 4) tend to take the helix dominative /3 form. For these molecules, the oxyethylene chain with approximately four or more OCH2CH2units is sufficiently long to form the intrinsic helical structure, as stated above. This helical nature of the oxyethylene chain seems to be strong enough to induce the helix incorporation of the adjoining CH2CH2CH20part of the alkyl chain. We found, however, a peculiar conformation for C12E7 which assumes the a form in place of the /3 form. There is nevertheless a possibility for this molecule to take the /3 form under other different conditions. One of possible factors to account for this peculiarity is that the seven units of OCH,CH, correspond exactly to two full turns of the 7, helix for poly(oxyethy1ene). However, we have no further evidence to characterize this conformational behavior. The discussions given above now establish the following conformational characteristics of the C,E, surfactants in the solid state. As the number of oxyethylene units ( m ) is increased, the conformation changes from the highly extended y form to the helix dominative /3 form. This conformational transition takes place around m = 3, which corresponds approximately to one turn of the 7, helix for poly(oxyethy1ene). On the other hand, the length of the alkyl chain does not seem to have so significant an influence on the conformational behavior of C,E, as that of the oxyethylene chain (see Table 11). The conformational behavior of the C,E, surfactants may be elucidated by the conformational competition between the oxyethylene chain which intrinsically fgvors the helical structure and the alkyl chain which favors the extended structure. (iii) Comparative Discussion with Previous Results. An early work by Rosch6 on the conformation of the oxyethylene chain in poly(oxyethy1ene) and a-n-alkyl-w-hydroxypoly(oxyethy1ene) (C,E,) has been cited by many authors in the field of surfactant chemistry. Although his X-ray diffraction experiments were very extensive, the results obtained were not necessarily authentic enough, since he used materials of heterogeneous chain length distribution. He presented two conformational models of the oxyethylene chain, Le., an extended zigzag conformation and a so-called meander conformation, and claimed that, on increasing average chain length, the former conformation is transformed into the latter at 20-40 oxyethylene units for polydisperse poly(oxyethylene) and a t 15-20 units for polydisperse C,E, surfactants in the bulk state.6 The meander model may be represented by the conformational sequence of gauche(O-CI)-skew(CI-CII)gauche(Clfl)-gauche'(O-CIII)skew'( CII+2rv)-gauche'(Cfl) where gauche and for -O-CIH2-CIIH2-O-CIIIHZ-CIvHz-O-, gauche' denote the conformational states with internal rotation angles in the vicinity of 60' and -60°, respectively, and skew and skew' denote those with internal rotation angles in the vicinity of 120' and -120°, respectively. The meander conformation seems to be an unlikely model in view of the rotational isomeric state,7 because it bears heavy steric hindrance and is thus estimated to be very unstable. The proposed meander model might have been replaced by the 7, helical conformation which had been established for p o l y ( o ~ y e t h y l e n e )before ~ ~ * ~ ~Rosch's article appearedG6The conformational transformation at m = 15-20 which he reported for the polydisperse C,E, surfactants is not consistent with our present result for the monodisperse materials. Many workers have referred to the meander model in elucidating their experimental results. We discuss briefly previous Raman spectroscopic investigations on nonionic surfactant^.^^^ Kalyanasundaram and Thomas8 examined the Raman spectra of polydisperse nonionic surfactants which included Igepal CO-630, Igepal CO-880, Brii 35, and Triton X-100. These surfactants c&&n oxyethylene chains with heterogeneous chain length distribution. They interpreted the spectra of these surfactants (42) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J . Chem. Phys. 1962.37, 2164-76. (43) Tadokoro, H.; Chatani, Y.; Yoshihara, T.; Tahara, S.; Murahashi, S . Makromol. Chem. 1964, 73, 109-27.
The Journal of Physical Chemistry, Vol, 91, No. 24, 1987 6147 in bulk (liquid or solid) and in aqueous solution on the basis of the 7, helical conformation of the oxyethylene chain5 and its disorder from the helix. These authors quoted Rosch's extended and meander conformation models6 but did not rely on these models. On the other hand, Cooney et aL9 studied the Raman spectra of alkylphenylpoly(oxyethy1ene) surfactants, namely, Triton X-100, Terics X-7, X-10, and X-13, and Terics N-2, N-5, and N- 10. Their spectral analysis for elucidating the molecular conformation was based on the assumption that there existed three basic forms: an extended zigzag conformation, a meander conformation, and a helical conformation. The first and second are the models proposed by Rosch,6 and the third corresponds to the 72 helix of p~ly(oxyethylene).~These authors made band assignments on the basis of a spectral comparison with related materials containing pertinent chemical groups. Unfortunately, however, some of their band assignments to particular groups were inadequate as noted in our previous paper,44making their conformational elucidation more or less fruitless. Particularly, the assignments of the 802- and 1098-cm-' bands of Triton X-100 to the extended zigzag conformation of the oxyethylene chain were irrelevant. These bands have been confirmed to be associated almost exclusively with the phenyl group but not with the oxyethylene An electron and X-ray diffraction study by Dorset'O on the crystal packing of monodisperse nonionic surfactants CsE, ( m = 1-5) and ClzE, ( m = 1-9) is highly instructive in understanding our present results. He found that C12E4 through C,& and CsE4 and CsE5 are structurally similar to one another but are different from the lower homologues. This finding is in nice agreement with our present results that the C,E, surfactants with m I 4 have the helix dominative /3 form (the CY form for CI2E7),while those with smaller m values take the highly extended conformation. He also pointed out a possibility for C12E3 to have two polymorphic forms. This polymorphism has now been confirmed in our Raman spectroscopic study, which showed the existence of the y and /3 forms depending on the solidification condition. The compounds of C12E, with m I 4 crystallize as spherulites,I0 similar to poly(o~yethylene).~~ This observation of the crystal growth habit supports the helical conformation of the oxyethylene chain involved in the structure of the /3 form. The behavior of the freezing point of the C12E, compounds with varying mlo may well be interpreted by the conformational transition at m = 3 . From the observed X-ray second-order long spacings for CI2Emand CsE,, Dorset'O concluded that for the materials with m 2 4 the alkyl chain is tilted by 25' away from the oxyethylene helix axis. The tilting of the alkyl chain is a necessary consequence of the molecular conformation for the /3 form; namely, the 7, helical conformation of the oxyethylene chain linked with the extended zigzag conformation of the alkyl chain geometrically requires the tilting, as is readily recognized if one constructs the molecular model (see Figure 8). The long-spacing increment from ClzEzto CIzE3was found to be larger than the corresponding increment from CI2E, to C12Em+lwith m 1 4.1° This difference in the increment is interpreted by a conformational consideration that, for CIZE2and C12E3, the oxyethylene chain takes the extended structure, while for C12E4 and the higher homologues, it takes the helical structure. The C,E, compounds we have treated in the present work are closely related to n-alkyl-oligo(oxyethy1ene)-n-alkyl triblock compounds CH3(CH2)n-1(OCH2CH,)mO(CHz)~,CH3. Booth and co-worker~~"~ have shown by X-ray scattering and infrared and Raman spectroscopy that, for the triblock compounds with nine or more oxyethylene units, the oxyethylene central block is in a (44) Matsuura, H.; Fukuhara, K. Chem. Lett. 1986, 191-4. (45) Price, F. P.; Kilb, R. W. J . Polym. Sci. 1962, 57, 395-403. (46) Domszy, R. C.; Booth, C. Makromol. Chem. 1982, 183, 1051-70. (47) Teo, H. H.; Swales, T. G. E.; Domszy, R. C.; Heatley, F.; Booth, C. Makromol. Chem. 1983, 184, 861-77. (48) Swales, T.G. E.; Domszy, R. C.; Beddoes, R. L.; Price, C.; Booth, C. J . Polym. Sei., Polym. Phys. E d . 1985, 23, 1585-95. (49) Swales, T.G. E.; Beddoes, R. L., Price, C.; Booth, C. Eur. Polym. J . 1985, 7, 629-34.
J . Phys. Chem. 1987, 91, 6148-6151
6148
in the experimental work. Thanks are also due to the Information Processing Center, Hiroshima University, and the Computer Center, Institute for Molecular Science, Okazaki National Research Institutes, for the use of their HITAC M-200H computers. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 60540289 from the Ministry of Education, Science, and Culture, Japan. Registry NO. 112-25-4; C6E2, 112-59-4; CgE1, 10020-43-6; CgE2, 19327-37-8; CsE,, 19327-38-9; CgE4, 19327-39-0; CIoEI, 2323840-6; CIOE2, 23238-41-7; CloEJ, 4669-23-2; CloE4, 5703-94-6; CloE,, 5168-89-8; CloEs, 24233-81-6; C12E1, 4536-30-5; C12E2, 3055-93-4; Cl2E3, 3055-94-5; C12E4, 5274-68-0; Cl2E5, 3055-95-6; Cl2E6, 3055-96-7; CI2E7, 3055-97-8; C12Eg, 3055-98-9; ClJEo, 112-70-9; CIdEo, 112-72-1; CisEo, 629-76-5; Cl6E4, 5274-63-5; C1&6, 5168-91-2; C16Eg15698-39-5.
helical conformation as in poly(oxyethy1ene) and the end alkyl blocks are in an extended zigzag conformation. This molecular conformation resembles the p form that we have found for the C,E, compounds. The helical conformation of the oxyethylene chain in these triblock compounds is expected from the conformational competition between the oxyethylene and alkyl chains, the oxyethylene chain in the central block being long enough to stabilize the helical structure. The tilting of the alkyl chains has also been established for the n-alkyl-oligo(oxyethy1ene)-n-alkyl compounds,4649 in conformity with the requirement by their molecular conformation.
Acknowledgment. We express our thanks to Dr. Keiichi Ohno for his helpful discussion and Mr. Toru Oyama for his assistance
Theoretical Determination of the Ground State of N,*+ Peter R. Taylor* ELORET Institute,t Sunnyvale, California 94087
and Harry Partridge N A S A Ames Research Center, Moffett Field, California 94035 (Received: April 13, 1987)
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The dication Nz2+is shown to have a IZ ground state, with a 311uexcited state less than 0.1 eV higher in energy. The lowest 3Z; state lies more than 0.5 eV a%ove the ground state. The computed T, for the DIE,+ X'Zg+ transition is in excellent agreement with experiment. The largest calculations performed include g-type basis functions and employ second-order CI expansions of up to 2 756 000 CSFs: the effect of selecting reference CSFs for the CI is also discussed. +
Introduction The dication N?+ is important in energetic processes involving molecular nitrogen, such as reactions in the ionosphere and in bow shock waves ahead of reentry vehicles. The available experimental results have been comprehensively reviewed and compared with theoretical calculations in a recent paper by Wetmore and Boyd.' The most accurately characterized data are derived from emission 'Z +) band ~ y s t e m . ~ , ~ spectroscopy of the Carroll-Hurley ('Z,+ While some theoretical studies have identified dZg+as the ground state of N22+,e6 including those of Wetmore and Boyd, suggest that the ground state is 311u. Earlier CASSCF studies by TaylorS accounted for near-degeneracy effects, but not dynamical correlation, and predicted a lZg+ground state. Wetmore and Boyd' employ a multireference CI treatment, which includes dynamical correlation, but use SCF M O s , which in the light of the CASSCF results5 may compromise the description of neardegeneracy effects. It is somewhat surprising that the calculations of Wetmore and Boyd predict a 311, ground state, as singlet states are generally lowered more by dynamical correlation than triplet states, and it is unusual to see a CASSCF singlet ground-state prediction reversed by inclusion of dynamical correlation. Few calculations on NzZ+have used extended 1-particle basis sets. The basis of Wetmore and Boyd' is very restricted (a [3s 2p Id] contracted Gaussian basis), and the basis set used by TaylorS is only slightly larger ([5s 3p Id]). Cossart and coworkers6 have used a Slater-type basis in a small C I calculation (yielding a IZg.+ ground state), and Cobb et a1.* have used more flexible Gaussian basis sets in S C F calculations, but the latter are very unreliable for N22+because of the severe near-degeneracy effects. It certainly seems desirable to perform CI calculations on N?+ in larger 1-particle basis sets than have been used to date. In the present work, we investigate the low-lying IZg+, 311u,and
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'Mailing address: NASA Ames Research Center, Moffett Field, CA 94035.
0022-3654/87/209 1-6148$01 .SO10
3Z,- states in the immediate region of the minima in their respective potential curves. As the results of high-resolution optical spectroscopy on N?+ are limited to the lZu+ IZ,+transition, we have also computed spectroscopic constants for the '2,' state for comparison with experiment. The aim of this work is not only to establish the ground state of Nz2+,but also to calibrate approaches for computing the entire potential curves of many states of Nz2+.
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Computational Methods Several different atomic basis sets have been used in this work. The smallest is a [Ss 3p Id] set given by Dunning's segmented contractiong of Huzinaga's (9s 5p) primitive setlo for N, augmented with a single d function with exponent 0.9. This is the basis used in earlier CASSCF studiesSand is similar to that used by Wetmore and Boyd,' but more flexibly contracted. The two larger basis sets are derived from the (13s 8p) primitive set of van Duijneveldt," augmented with a (6d 4f 2g) polarization set. The polarization functions are taken as even-tempered sequences with an internal ratio of 2.5. The geometric mean of the d exponents is 1.0,l2while the f and g sets are based on scaling of the mean d exponent by 1.2 and 1.44, respectively. This primitive (1) Wetmore, R. W.; Boyd, R. K. J. Phys. Chem. 1986, 90, 5540. (2) Carroll, P. K. Can. J . Phys. 1958, 36, 1585. Carroll, P. K.; Hurley, A. C. J. Chem. Phys. 1961, 35, 2247. (3) Hurley, A. C. J. Mol. Specirosc. 1962, 9, 18. (4) Thulstrup, E. W.; Andersen, A. J. Phys. B 1975, 44, 285. (5) Taylor, P. R. Mol. Phys. 1983, 49, 1297. (6) Cassart, D.; Launay, F.; Robbe, J. M.; Gandara, G. J . Mol. Spectrosc. 1985, 113, 142. (7) Stalherm, D.; Cliff, B.; Hillig, H.; Mehlhorn, W. Z . Naturforsch., A. 1969, 24, 1728. ( 8 ) Cobb, M.; Moran, T. F.; Borkman, R. F.; Childs, R. J . Chem. Phys. 1980, 72, 4463. (9) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823. (10) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (1 1 ) van Duijneveldt, F. B. ZBM Res. Rep. 1971, RJ 945. (12) Ahlrichs, R.; Taylor, P. R. J . Chim. Phys. 1981, 78, 1413.
0 1987 American Chemical Society
