6638
J . Phys. Chem. 1989, 93, 6638-6642
Vibrational Spectra of N-Acylglyclne Oligomers. 1. N-Acetyl Derivatives and Helical e Structure Conversion Hirofumi Okabayashi,* Kunihiro Ohshima, Hideki Etori, Keijiro Taga, Tadayoshi Yoshida, Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa- ku, Nagoya 466, Japan
and Etsuo Nishio Perkin- Elmer Japan, Osaka Laboratory, Miyahara, Yodogawa- ku, Osaka 532, Japan (Received: February 2, 1989)
N-Acetylglycine trimer, tetramer, pentamer, their potassium salts, and their N-deuterated derivatives have been prepared. Two crystalline modifications (solid-A and solid-B) have been obtained for acid-type molecules. The solid-A is more stable than the solid-B at room temperature. The solid-A Q solid-B conversion similar to the polyglycine I1 (PGII) * polyglycine I (PGI) conversion has been observed. The X-ray powder patterns and the vibrational spectra of these compounds have been measured and compared with those of PGI and PGII. For acid-type molecules, the solid-A is in the PGII-like structure (helical form) and the solid-B is in the FGI-like structure (/3 form). In the case of potassium salts, only the PGII-like structure exists in the solid state and the conformation similar to PGII predominantly exists in aqueous solution.
Introduction Polyglycine (PG) exists in two crystalline forms known as I and 11.’ According to the X-ray studies, PGI was in the /3 structure2” and PGII in the helical s t r u ~ t u r e . ~ The ’ vibrational spectra of PGI and PGII also have been investigated in detail by many authors.8-21 In the case of glycine oligomers, Smith et aLi3have concluded that glycine through pentaglycine are in the PGI-like structure from the Raman spectral analysis. Additionally, Dwivedi et a1.22 have indicated in an IR study that hexaglycine has the PGI-like structure, while dodecaglycine takes the PGII-like structure. Gupta et aLz3have also reported by the dispersion curve analysis that oligomers up to the pentamer show a similarity to the PGI-like structure whereas the transition from the PGI-like structure to the PGII-like one starts at the hexamer and a complete helix is formed at the dodecamer. (1) Bamford, C. H.; Brown, L.; Cant, E. M.; Elliot, A,; Hanby, E. W.; Malcolm, B. R. Nature 1955, 176, 396. (2) Astbury, W . T.; Dalgliesh, C. E.; Darmon, S. E.; Sutherland, G. B. 8. M. Nature 1948, 162, 596. (3) Astbury, W. T. Nature 1949, 163, 722. (4) Crick, F. H. C.; Rich, A. Nuture 1955, 176, 780. (5) Ramachandran, G. N.; Sasisekharan, V.; Ramakrishnan, C. Biochim. Biophys. Acta 1966, 112. 168. (6) Krimm, S. Nature 1966, 212, 1482. (7) Ramachandran, G. N.; Ramakrishnan, C . ; Venkatachalam, C. M. In Conformation of Biopolymers; Ramachandran, G. N., Ed.; Academic Press: New York, 1967; p 429. (8) Fukushima, K.; Ideguchi, Y.; Miyazawa, T. Bull. Chem. SOC.Jpn. 1963, 36, 1301. (9) Suzuki, S.;Iwashita, Y.; Shimanouchi, T.; Tsuboi, M. Biopolymers 1966, 4, 337. (IO) Miyazawa, T. In Poly-a-Amino Acids; Fasman, G. D., Ed.; Dekker: New York, 1967; p 69. ( 1 I ) Gupta, V. D.; Trevino, S.;Boutin, H. J. Chem. Phys. 1968, 48, 3008. (12) Krimm, S . ; Kuroiwa, K. Biopolymers 1968, 6, 401. ( 1 3) Smith, M.; Walton, A. G.; Koenig, J. L. Biopolymers 1969, 8, 29. (14) Small, E. W.; Fanconi, B.; Peticolas, W. L. J . Chem. Phys. 1970,52, 4369. ( I 5) Singh, R. D.; Gupta, V. D. Spectrochim. Acta, Part A 1971, 27, 385. (16) Abe, Y . ; Krimm, S. Biopolymers 1972, 11, 1817. (17) Abe, Y . ; Krimm, S. Biopolymers 1972, 11, 1841. (18) Fanconi, B. Biopolymers 1973, 12, 2759. (19) Moore, W. H.; Krimm, S. Biopolymers 1976, I S , 2439. (20) Dwivedi, A. M.; Krimm, S. Macromolecules 1982, 15, 177. (21) Dwivedi, A. M.; Krimm, S.Biopolymers 1982, 21, 2377. (22) Dwivedi, A. M.; Gupta, V. D. Chem. Phys. Lett. 1971, 8 , 220. (23) Gupta, V . D.; Gupta, M. K.; Nath, K. Biopolymers 1975, 14, 1987.
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Conformational studies of the glycine oligomers have been made to determine the critical size for the formation of secondary structure of polypeptide. Koyama et aLZ4have studied the conformation of N-acetylglycine methylamide by the IR spectra and discussed short-range factors determining the conformation of the glycine peptide skeleton. Avignon et have reported that oligomers of N-acetylglycine ethylamide, C H 3 C O (NHCH2CO),NHC,H5 (n = 1-4), take the helical structure in the solid state from the IR study. This paper presents the conformational study of N-acetylglycine oligomers, CH3CO(NHCH,CO),0H ( n = 3-5), and their potassium salts in the solid state and aqueous solution. Their conformation is discussed on the basis of comparison of the vibrational spectra and the X-ray diffraction powder patterns with those of PGI and PGII.
Experimental Section Materials. N-Acetylgycine trimer (AcG3H) was prepared by the method similar to that used by Herbst et aI.% N-Acetylglycine tetramer (AcG4H) was prepared from the solvolysis and acidification of the ethyl ester of N-acetyltetraglycine. The ethyl ester of N-acetyltetraglycine was prepared by the condensation of N-acetyltriglycine and glycine ethyl ester by a method similar to that described by Ionova et aL2’ N-Acetylglycine pentamer (AcGSH) was prepared in the same manner by use of glycylglycine ethyl ester. Two crystalline modifications were obtained as follows. NAcetylglycine oligomers were dissolved in saturated LiBr aqueous solution and treated with active charcoal. The mixture was allowed to stand overnight, and then water was added. The resulting precipitate was filtered and washed with water several times and dried (A-series samples: solid-A, AcGH-A). The A-series samples were dissolved in dichloroacetic acid (DCA), and then the solvent was removed by evaporation in a vacuum system. The crystalline substances obtained were washed with dry ether several times and dried (B-series samples: solid-B, AcGH-B). N-Deuterated Aseries (AcGD-A) and B-series (AcGD-B) samples were prepared by the treatment in LiBr-D20 or 0-deuterated DCA. Potassium salts of these oligomers were prepared by neutralization of the acid-type molecules and potassium hydroxide in ~
~~
(24) Koyama, Y . ;Shimanouchi, T. Biopolymers 1968, 6, 1037. (25) Avignon, M.; Garrigou-Lagrange, C. Spectrochim. Acta, Part A 1971, 27, 297. (26) Herbst, R. M.; Shemin, D. Organic Syntheses; Blatt, A. H., Ed.; Wiley: New York, 1966; Collect. Vol. 2, p 1 1. (27) Ionova, L. V.; Mozzhukhin, D. D.; Morozova, E. A. J . Gen. Chem. USSR (Engl. Transl.) 1964, 34, 768.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6639
Vibrational Spectra of N-Acylglycine Oligomers
3500
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Figure 1. Raman spectra of (A) the A-series of undeuterated N-
acetylglycine oligomers (a) AcG~H-A,(b) AcG~H-A,and (c) AcGSHA, and (B) their N-deuterated oligomers (a) AcG3D-A, (b) AcG4D-A, and (c) AcGSD-A, at room temperature in the solid state. Asterisks mark the bands corresponding to the characteristics of PGII. methanol. N-deuterated potassium salts were prepared by the D20 treatment. All the A-series samples were identified by elemental analysis. Anal. Calcd for AcG3H-A (CsHI3O5N3):C, 41.56; H, 5.67; N, 18.17. Found: C, 41.39; H, 5.65; N, 17.75. Anal. Calcd for AcG4H-A (CIOHI606N4):c , 41.67; H, 5.59; N, 19.44. Found: C, 41.23; H, 5.46; N, 19.06. Anal. Calcd for AcG5H-A (CI2Hl9O7N5):C, 41.74; H, 5.55; N , 20.28. Found: C, 41.55; H, 5.59; N, 20.35. Polyglycine was obtained from Sigma Chemical Co. and was purified by the method of Small et al.I4 Raman Scattering Measurements. All the Raman spectra were taken with a Jeol Model JRS-400D Raman spectrometer, with 5 14.5-nm excitation of an NEC GLS-3200 argon ion laser at room temperature. Infrared Absorption Spectrum Measurements. A Perkin-Elmer 1700 Fourier transform infrared spectrometer (400-4000 cm-l) and a Hitachi FIS double-beam grating spectrometer (50-400 cm-I) were used. A circle cell (ZnSe; Barnes/Spectra-Tech Inc.) was used for the measurements in aqueous solutions. X-ray Diffraction Powder Pattern Measurements. The X-ray diffraction powder patterns were obtained by use of a RAD-RC diffractometer with countermonochrometer (Cu Ka ray; voltage, 60 kV; current, 200 mA). Abbreviations for Vibrational Assignment. The following abbreviations are used for these oligomers: amide I, mainly C = O stretching vibration; amide 11, N H in-plane bending vibration coupled with amide C N stretching; amide 111, mainly amide C N stretching vibration.
Results and Discussion ( I ) X-ray Diffraction Powder Patterns of N-Acetylglycine Oligomers. The X-ray diffraction powder patterns of Nacetylglycine oligomers were measured and compared with those of PGI and PGII.'-' The patterns of the A-series were different from those of the B-series. For the A-series, very intense reflections
4000
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2400 1600 Wavenumber / cm-l
800
Figure 2. FT-IR spectra of A-series of undeuterated N-acetylglycine oligomers in the solid state at room temperature: (a) AcG3H-A; (b) AcG4H-A; (c) AcGSH-A. Asterisks mark the bands corresponding to the characteristics of PGII.
at 4.18-4.20 A and weak ones at 3.12-3.17 A were observed, corresponding to the 4.14- and 3.09-8, reflections of PGII, respectively, while the patterns of the B-series contained very intense reflections at 3.36-3.37 8, and medium ones at 4.35-4.39 A, corresponding to the 3.42- and 4.36-8, reflections of PGI, respectively. This suggests that the A-series oligomers take the PGII-like structure and the B-series the PGI-like structure. Potassium salts gave only the PGII-like reflections. ( 2 ) Vibrational Spectra of N-Acetylglycine Oligomers. ASeries. The Raman and FT-IR spectra of A-series of undeuterated N-acetylglycine oligomers are shown in Figures 1 and 2. Figure 1B shows the Raman spectra of A-series of N-deuterated oligomers. The vibrational bands of AcGH-A in the CH2-stretch region are very similar to those of PGII rather than those of PGI in f r e q u e n ~ y . ~ , ' ~The * ~ ~IR * ~ (Raman) ~ bands at 2976-2980 (2978-2981) and 2939-2943 (2940-2943) cm-' and the IR bands at 2852-2854 and 2810 cm-' are very close to those at 2977 (2979) and 2935 (2940) and 2850 and 2805 cm-I for PGII. The CH2 stretch modes of AcGD-A show the spectral pattern similar to those of AcGH-A (Figure 1A,B). The strong Raman bands at 1652-1653 and 1607-1625 cm-' for AcGH-A are assigned to the amide I modes. The former bands closely correspond to the band at 1654 cm-' for PGII, while the latter bands were not observed for PGII and may be regarded as the amide I bands characteristic of these oligomers. For the IR spectra of AcGH-A, the amide I mode splittings are observed at 1649-1 65 1 and 1631-1 635 cm-I, corresponding to those at 1655 and 1640 cm-' for PGII, respectively. The amide I split separation , = 14 in the IR spectra of AcGH-A is as follows: A c G ~ H - AAv cm-I; A c G ~ H - A ,Av = 19 cm-'; AcG5H-A, Au = 16 cm-I. The
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The Journal of Physical Chemistry, Vol. 93, No. 18, 1989
Okabayashi et al.
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Raman spectra of (A) the B-series of undeuterated Nacetylglycine oligomers (a) AcG~H-B,(b) AcG~H-B,and (c) AcGSH-B, and (B) their N-deuterated oligomers (a) AcG~D-B,(b) AcG~D-B,and (c) AcGSD-B, at room temperature in the solid state. Asterisks mark the bands corresponding to the characteristics of PGI. Figure 3.
deuterium substitution of the N H groups causes the splittings in the amide I region of AcGD-A. The Raman spectral feature in the amide I region depends upon the residue number (Figure 1B). The strong IR band at 1558 cm-l common to each AcGH-A is assigned to the amide I1 mode and is very close to the band at 1550 cm-I for PGII rather than that at 1517 cm-I for PGI. On deuteration of the N H groups, the amide I1 band shifts to 1476-1 483 cm-' for AcGD-A. The amide 111 mode appears at 1281-1296 (Raman) and 1281-1284 cm-I (IR) and corresponds to the Raman and IR bands at 1283 cm-I for PGII. These bands shift to 990-997 (Raman) and 985-987 cm-I (IR) by deuteration of the N H groups and correspond to the bands at 995 (Raman) and 987 cm-I (IR) for PGII(ND). The Raman bands at 1135-1140 and 1034-1037 cm-I are assigned to the skeletal stretch modes for AcGH-A. The split separation of the skeletal stretch modes (Au = 101-104 cm-I) is very close to that of Av = 103 cm-I for PGII (Raman bands at 1134 and 103 1 cm-l) rather than that of Au = 143 cm-l for PGI (Raman bands at 1164 and 1021 cm-I). Below the 900-cm-* region, the spectral pattern of AcGH-A is almost the same as that of AcGD-A (Figure lA,B). The prominent Raman bands at 886-887 (875) cm-' for AcGH-A (AcGD-A) are assigned to the CHI rock and are also characteristic of PGII and PGII(ND). The 1R band at 363 cm-I for PGII arising from the skeletal deformation mode reflects directly the conformational difference between PGI and PGII. The bands at 354-360 cm-I for AcGH-A show the similarity of the skeletal structure of the A-series to PGII (Figure 5). B-Series. Figures 3A and 4 show the Raman and IR spectra of the B-series of undeuterated N-acetylglycine oligomers, respectively. The Raman spectra of the B-series of N-deuterated oligomers are shown in Figure 3B. The calculated frequencies of PGI(ND)20were used for the assignment of AcGD-B, since no Raman spectra of PGI(ND) were reported.
I .f
m
2 4000
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Figure 4. FT-IR spectra of B-series of undeuterated N-acetylglycine oligomers in the solid state at room temperature: (a) AcG3H-B; (b)
AcG4H-B; and (c) AcGSH-B. Asterisks mark the bands corresponding to the characteristics of PGI. For AcGH-B, the Raman bands at 2928-2930 and 2864-2865 cm-I and the corresponding IR bands at 2927-2936 and 2875 an-] assigned to the CHI stretch are very similar to those for PGI rather * ~ Raman ~ * ~ ' bands at than those for PGII in f r e q u e n ~ y . ~ * ' ~The 2955-2958 cm-l for AcGH-B are also the characteristic bands of PGI-like structure. Parts A and B of Figure 3 show that AcGH-B and AcGD-B have similar spectral patterns in the CH2 stretch region. The Raman bands at 1659-1666 and 1625-1644 cm-' are assigned to the amide I mode in AcGH-B. The strong bands at 1659-1666 cm-I for the B-series are a t a higher frequency side than those at 1652-1653 cm-I for the A-series and come close to the amide I band of PGI at 1674 cm-I with increasing residue number. The IR bands at 1680-1685 and 1633-1638 cm-' are also assigned to the amide I modes and correspond to the characteristic bands of PGI at 1685 and 1636 cm-', respectively. The amide I split separation in the IR spectra of AcGH-B is as follows: , = 44 cm-'; AcG5H-B, A c G ~ H - B Au , = 42 cm-'; A c G ~ H - B Au Av = 52 cm-I. These are considerably different from Av = 14-19 cm-I AcGH-A. The split separation of AcGH-B comes close to that for PGI (49 cm-I) with an increase in residue number. The amide I mode splitting for AcGD-B also occurs similar to that of AcGH-B. The bands at 1520-1522 (Raman) and 1518-1523 cm-I (IR) assigned to the amide I1 mode vanish upon N-deuteration and shift to 1489-1496 (Raman) and 1480-1483 cm-I (IR). The amide I1 bands of AcGH-B are obviously different from that of AcGH-A at 1558 cm-l (IR) and are very close to those at 1515 (Raman) and 1517 cm-I (IR) for PGI. The Raman bands for AcGH-B at 1154-1 158 and 1014-1016 cm-I correspond to those at 1164 and 1021 cm-' for PGI and are
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6641
Vibrational Spectra of N-Acylglycine Oligomers
TABLE I: Observed Frequencies (em-') of PGII-Characteristic Bands for N-Acetylglyclne Oligomer K Salts in the Solid State and Aqueous Solutions'b AcG~K-AcG~K AcG4K( ND), AcG5K( ND) solid aqueous solution solid Raman IR Raman IR Raman 1655-1658 (s) 1641-1657 (VS) 1657-1659 (s, b) 1645-1650 (m) 1639-1641 (s) 1555-1560 (VS) 1552-1553 (VS) 1475-1476 (m) 1422-1427 (s) 1417-1421 (s) 1417-1427 (s) 1413-1419 (m, sh) 1421-1422 (m) 1384-1385 (s) 1391-1392 (s) 1394-1395 (s) 1395 (ms) 1284-1285 (m) 1297-1299 (s) 1284-1291 (m) 1285-1290 (m,sh) 1270-1273 (m) 1257-1264 (m) 1251-1254 (m) 1263-1264 (s) 1261-1265 (m) 1135-1140(~) 1124-1128 (w) 1030-1031 (w) 1032-1033 (VS) 1032 (w) 1032-1033 (w) 1034-1035 (s) 1010-1020 (VW,sh)' 994-996 (VS) 887-893 (m) 871-875 (s) 878 (vs) 698-706 (m) 576-577 (w) 561-564(~) 'Only the main vibrational bands are listed. *Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; b, broad. cPGI-characteristic band.
I
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300
200
400 Wavenumber / cm-'
100
300
200
100
Figure 5. Far-IR spectra of A- and B-series of undeuterated N-acetylglycine oligomers: (A) (a) AcG3H-A, (b) AcG~H-A,and (c) AcG5HA; (B) (a) AcG3H-B, (b) AcG~H-B,and (c) AcG5H-B. Asterisks mark the bands corresponding to the characteristics of PGII and PGI.
assigned to the skeletal stretch modes. These characteristic bands are also observed for AcGD-B. Below the 800-cm-' region, the spectral feature of AcGH-B is almost the same as that of AcGD-B (Figure 3A,B). Figure 5 also shows the IR spectra of the skeletal deformation vibration region for AcGH-B. The IR bands at 317, 221, and 134 cm-' fnr AcG3H-R a t 31R and 173 rm-1 fnr A r G d H - R and a t 212
for ACWH-B are also charactenstic or -1-like structure. These observations indicate the similarity of the skeletal structure of the B-series to PGI. ( 3 ) C=O Stretch Modes of COOH in Oligomers. The Raman and IR bands at 1740-1 742 cm-' for AcGH-A are assigned to the C=O stretch mode of end COOH, which are weakly hydrogen-bonded but not cyclic-dimerized.28 The IR bands at 1707-1 7 14 cm-' are due to the asymmetric stretch mode of cyclic COOH dimers, while the Raman bands at 1670-1676 cm-l arise from the C=Q symmetric stretch of the cyclic dimers. Thus, both the cyclic dimers and weakly hydrogen-bonded types of the COOH groups coexist in the A-series of oligomers (Figures 1A and 2), while for AcGH-B the C=O stretch mode of end COOH appears at 1747-1749 (Raman) and 1749-1750 cm-' (IR). This obserLLU, ana I jj cm-'
(28) Haurie, M.; Novak, A. J . Chim. Phys. Phys.-Chim. Biol. 1965, 62,
146.
vation shows that only the weakly hydrogen-bonded type of the COOH groups exists in the B-series of oligomers (Figures 3A and 4). ( 4 ) Stability of A- and B-Series of Oligomers in the Solid State. The FT-IR spectra of the A-series and B-series samples were measured in the solid state at monthly intervals. When the B-series samples were kept for two months in a desiccator at room temperature, the PGII-characteristic bands at 1551, 1373-1 375, 1284-1286, 1252, and 1030-1032 cm-' were newly observed in addition to the PGI-characteristic bands. This observation shows that the conformational change from the PGI-like structure to the PGII-like one occurs for the B-series samples, while for the A-series samples the PGI-characteristic bands did not appear in the same condition. Thus, the PGI-like structure for the B-series samples are less stable than the PGII-like structure for the A-series samples in the solid state. ( 5 ) Conformations of Potassium Salts of N-Acetylglycine Oligomers. Table I gives the ObServed vibrational wavenumbers for N-acetylglycine oligomer potassium salts in the solid state and aqueous solutions. The same abbreviation as the A-series is used for these molecules, and K denotes the potassium salt. The amide I mode in the Raman (IR) spectra at 1655-1658 (1641-1657) cm-' is very close to that of PGII at 1654 cm-' rather than that of PGI at 1674 cm-'. The IR frequencies of the amide I band shift to the lower frequency side with increasing residue number and come close to that of PGII (1658 cm-l for AcG3K and A&4K and 1655 cm-' for AcG5K). The amide I1 IR bands at 1555-1560 cm-' for the K salts are significantly different from that at 1517 cm-' for PGI and closely correspond to that of PGII at 1550 cm-I. The PGII-characteristic bands are also found below the 1500-cm-' region. In particular, the bands at 1135-1 140, 1034-1035 (Raman), and 1030-1031 cm-' (IR) arise from the skeletal stretch modes, corresponding to the characteristic band of PGII at 1134 (1132) and 1031 (1028) cm-' in the Raman (IR) spectra. The PGII-characteristic bands are also observed for N-deuterated derivatives (Table I). In the case of aqueous solution, the Raman bands at 1417-1427, 1263-1264, and 1032-1033 cm-' correspond to those a t 1421, 1261, and 1031 cm-' for FGII and the difference FT-IRbands at 1552-1553 and 1032 cm-' to those at 1550 and 1028 cm-' for PGII. These observations show that the conformation similar to PGII predominantly exists in aqueous solution. The ionized carboxyl groups must play a critical role in the stabilization of the helical structure, as Shoemaker et al.29.30have pointed out the (29) Shoemaker, K. R.; Kim, P. S.; Brems, D. N.; Marquee, S.; York, E. J.; Chaiken, I. M.; Stewart, J. M.; Baldwin, R. L. Proc. Narl. Acud. Sci. U.S.A. 1985, 82, 2349.
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importance of the charged groups for the stability of the C-peptide helix. The very weak IR shoulder bands at 1010-1020 cm-l are also observed in aqueous solutions. This indicates that the PGI-like structure of the K salts coexist in aqueous solution, although the population is so small.
Conclusions Acid types of N-acetylglycine trimer, tetramer, pentamer, and their N-deuterated molecules have two crystalline modifications ( 3 0 ) Shoemaker, K. R.; Kim, P. S.; York, E. J.; Stewart, J. M.; Baldwin,
R. L. Nurure 1987, 326, 563.
of solid-A and solid-B. The solid-A is found to have the PGII-like (helical form) structure and the solid-3 to have the PGI-like ( p form) structure. The helical w p structure conversion analogous to polyglycine (PGII w PGI conversion) is possible for these molecules, while their potassium salts take only the PGII-Iike structure in the solid state. Thus, the end COOH groups of these oligomers must play an important role in the solid A 0 solid B conversion. For the potassium salts, the PGII-like structure is also preferentially stabilized in aqueous solution. Acknowledgment. We express our thanks to Prof. M. Abe of Showin Women's University for her helpful discussion.
Isotope Effect on the Static Distortion and the Dynamics of Partially Deuteriated Jahn-Teller Active Radical Cations: Cyclopropane- I , I-d2 K. Matsuura, K. Nunome, M. Okazaki, K. Toriyama,* and M. Iwasakit Government Industrial Research Institute, Nagoya, Hirate. Kita, Nagoya 462, Japan (Received: February 15, 1989)
An isotope effect in the Jahn-Teller (J-T)distortion has been found for the partially deuteriated cyclopropane radical cation (cyclopropane-lJ-d,') formed in the frozen matrices at 4 K. The cation radical distorts from a regular triangle with D3h symmetry into an obtuse triangle with Cb symmetry. The singly occupied molecular orbital (SOMO) is found to be 6a1, as is the case of the protiated cyclopropane radical cation. In the deformed cyclopropane-l,l-d,+, the CD2 group tends to occupy the top of the obtuse triangle selectively. Moreover, the barrier of the J-T potential trough increased by the partial deuteriation. The result is discussed in relation to the J-T distortion of partially deuteriated ethane radical cations.
Introduction In a previous paper,' we have reported direct evidence for the deuterium isotope effect on the static distortion on Jahn-Teller (J-T) active radical cations as well as on the activation energies for their dynamic process. That is, the radical cations of partially deuteriated ethanes (C2H6,D,+) exhibit C,,,distortion in SF6at 4 K to have the *A, state, as is the case of the protiated ethane radical cation, c&.+.2In this structure, two in-plane C-H bonds tilt toward the upright position with respect to the C - C bond. In partially deuteriated ethane radical cations, the in-plane upright position is occupied preferentially by a C-H bond as far as H is involved in the methyl group. In addition, the activation energy needed to average the distorted structures becomes higher when the C3 symmetry of the methyl group is lost by the partial deuteriation.' A similar drastic deuterium substitution effect has also been found by Knight et al. on CH2D2+in the Ne m a t r i ~ . In ~ this case, C-H bonds again preferentially occupy the bonds with larger distortion. In the present work, we studied the deuterium substitution effects on the static distortion and the dynamics of partially deuteriated cyclopropane radical cations, cyclopropane-I ,I -d2+, since it is another important J-T active molecule. Moreover, it is of interest to know how deuterium substitution affects the J-T distortion of cyclopropane+where the ring deformation is the major mode of distortion. It has been found in our previous work that the radical cation of protiated cyclopropane distorts from the D3,, symmetry of the mother molecule into C, symmetry and the stable form in freons or SF6 at 4 K is an obtuse triangle with a 6al S O M O (Figure la).4vs In this cation radical, most of the spin is located on the two basal carbons. In cyclopropane-l,l-d,+, we can expect two kinds of isotopic positional isomers if they have the 6al SOMO. They are an obtuse triangle with the CD, group at the top (structure I in Figure Ib) and the one with the CD2 Deceased in June 1987.
0022-3654/89/2093-6642$01.50/0
group at the basal position (structure I1 in Figure lb). If there is any site preference of isotopes, the relative amount of these two isomers will deviate from the statistically expected value of 1:2. A fairly large isotope effect was found in the J-T distortion of cyclopropane-I ,l-d2+,although the SOMO was not changed by the partial deuteriation. An increase of the activation energy for the dynamic J-T effect was also found.
Experimental Section Partially deuteriated cyclopropane ( 1,1-dideuteriocyclopropane) was obtained from MSD Isotopes (Canada); protiated cyclopropane and SF6 were obtained from Takachiho Kogyo (Japan); CFC13 was obtained from Tokyo Kasei Kogyo (Japan), CFC1,CFzCl from Daikin Kogyo (Japan), and CF3CC13from Aldrich Chemicals. All the samples were used as received. The frozen solutions of cyclopropane in sF6and in freons were prepared by quenching liquid solutions containing 0.1-0.3 mol % of solute alkane at 77 K. In the case of CFC13 matrix, solid solution was crushed in a liquid nitrogen bath to get powdered samples, since the sample frozen in a sample tube sometimes shows partial orientation of crystallites in the ESR sample tube. Cation radicals were generated by X-ray irradiation (45 kV, 40 mA) at 4 K for about 15 min, and ESR measurements were performed at 4 K without raising the sample temperature or at higher temperatures. The experimental setup for irradiation at the cryogenic temper(1) Iwasaki, M.; Toriyama, K. Chem. Phys. Leu. 1984, I l l , 309. (2) (a) Iwasaki, M.; Toriyama, K.;Nunome, K. J . A m . Chem. SOC.1984, 106,3700. (b) Toriyama, K.; Nunome, K.; Iwasaki, M. J. Chem. Phys. 1982, 77., 5891. - -(3) Knight, Lon 9.; Jhobe, S . J . A m . Chem. SOC.1983, 105, 202. (4) Iwasaki, M.; Toriyama, K.; Nunome, K. J . Chem. SOC.,Chem. Commun. 1983, 202. ( 5 ) Iwasaki, M.; Toriyama, K.; Nunome, K. Furuduy Discuss. Chem. SOC. 1984, 78, 19.
0 1989 American Chemical Society
