Rotational Isomerism in Ethylamine
803
more reactive toward quasi free electrons than the higher. It would be interesting to study this reaction in the gas phase as a function of temperature. Acknowledgments. This work was supported by the U. S. Atomic Energy Commission. The authors are grateful to Dr. H. A. Schwarz for formulation of computer programs and for frequent helpful discussions. Thermoluminescent dosimetry was done by R. L. Gardner of the Brookhaven Health Physics Division. References and Notes (1) W. F. Schmidt and A. 0.Allen,J. Chem. Phys., 52, 4788 (1970). (2) R. M. Minday, L. D. Schmidt, and H. T. Davis, J. Chem. Phys., 54, 3112 (1971). (3) J. P. Dodelet and G. R. Freeman, Can. J. Chem., 50, 2667 (1972). (4) R. M. Minday, L. D. Schmidt, and H. T. Davis, J. Phys. Chem., 76, 442 (1972). (5) G. Bakale and W. F. Schmidt, A. Naturtorsch. A, 26, 511 (1973). (6) H. T. Davis, L. D. Schmidt, and R. M. Minday, Chem. Phys. Lett., 13, 413 (1972). (7) (a) H. A. Gillis, N. V. Klassen, G. G. Teather, and K. H. Lokan,
(8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
Chem. Phys. Lett., 10, 481 (1971); (b) L. 8. Magnusson, J. T. Richards, and J. K. Thomas, Int. J. Radiat. Phys. Chem., 3, 295 (1971). J. H. Baxendale, C. Bell, and P. Wardman, Chem. Phys. Lett., 12, 347 (1971). J. M. Warman, K. D. Asmus, and R. H. Schuler, Advan. Chem. Ser., No. 81, 25 (1968). G. Bakaie, E. C. Gregg, and R. D. McCreary, J. Chem. Phys., 57, 4246 (1972). G. Beck and J. K. Thomas, J. Chem. Phys., 57, 3649 (1972); J. Chem. Phys., submitted for publication. D. E. Hudson, United States Atomic Energy Commission Report NO. MDDC-524 (1946). P. Langevin, C. R. Acad. Sci., 134, 533 (1902). J. W. Boag, Brit. J. Radioi., 23, 601 (1950). P. Debye, Trans. Nectrochem. Soc., 82, 265 (1942). J. H. Sharp and M. Smith in "Physical Chemistry, an Advanced Treatise," Vol. IO, Solid State, W. Jost, Ed., Academic Press, New York, N Y , 1970, p 494. K. Fueki, 13.-F. Feng, and L. Kevan, Chem. Phys. Lett., '13, 616 (1972). N. F. Mott and, E. A. Davis, "Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1971, Chapter 2 and p 209. L. G. Christophorou, "Atomic and Molecular Radiation Physics," Wiley-interscience, New York, N. Y., 1971, Chapter 6. S. A. Hoiroyd and M. Alien, J. Chem. Phys., 54, 5014 (1971): R. A. Holroyd, ibid., 57, 3007 (1972).
Gauche-Trans Rotational Isomerism in Ethylamine. The Far-Infrared Spectra of CH3CH2ND2 and CH3CD2ND2' A. S. Manocha, E. C. Tuazon," Department of Chemistry, Carnegie-Melion University, Pittsburgh, Pennsylvania 15273
and W. G. Fateley Department of Chemistry, Kansas State University, Manhattan, Kansas 66502
(Received November 26, 7973)
The gauche-trans rotational isomerism of ethylamine in the vapor phase has been studied by an analysis of the far-infrared spectra of CH3CHzNDz and CH3CDzNDz. The potential function for internal rotation about the C-N bond, derived through a one-dimensional approximation, is 2V(a) = -303.2(1 - cos a ) + 184.4(1 - cos 2a) 734.7(1 - cos 3a) - 19.3(1 - cos 6a)cm-l. The difference between the potential energy minima of the gauche and trans conformations is 104 cm-1, the gauche being the more stable form.
+
Introduction Internal rotation about the C-N bond in ethylamine gives rise to two distinct stable isomers, the trans and gauche forms. The rotation of the amino group about the C-N axis is geometrically asymmetric. The asymmetry will be reflected in the form of the potential function governing the motion. It has been pointed out by LoweZathat the study of molecules with asymmetric internal rotors will provide more information concerning the origins of barrier to internal rotation than those obtainable from molecules with highly symmetric rotors (e.g., CH3 group). Recently, Radom, Hehre, and Poplezb predicted rotational barriers and conformational energy differences ih several molecules, including ethylamine, from ub initio molecular orbital theory. They related physical effects such as staggered arrangements of bonds, stabilization due to electron donation and electron withdrawal in bonds and
orbitals, and relative directions of dipole moment components to components of the potential function expressed as a Fourier series in the internal angle. In this work, the gauche-trms isomerism of ethylamine is studied by an analysis of tdhe observed frequencies belonging to the torsional motion of the amino group. The simpler vapor-phase far-infrared spectra of the deuterated species CH~CHZNDZ and CH~CDZNDZ, as compared to that of the light molecule CH3CHzNH2, enabled a good approximate potential function to be derived through a one-dimensional model. Experimental Section CHaCHzNDz was generated from a sample of CH3CHzNDz.DCl (obtained by direct exchange of CH~CHZNHZ .HC1 with Dz0) by treatment with a freshly ignited sample of CaO. C H ~ C D Z N Dwas ~ prepared The Journal of Physical Chemistry, Voi. 78, No. 8, 1974
A. S. Manocha, E. C. Tuazon, and W. G .
804
through reduction of CH3CN by LiAlD4. The samples prepared were thoroughly dried by passing through a column of preheated 3 A molecular sieves. The purity of the compounds was checked by their mid-infrared spectra. The .far-infrared spectra were recorded on a Digilab FTS-14 Fourier transform spectrometer operating in the double-beam mode. A 6 - p mylar beam splitter was used to select the range of frequency below 400 cm-l. A black polyethylene filter was also used to remove the unwanted higher-frequency output of the mercury-lamp source. The interferograms were transformed through a triangular apodization function. The maximum resolution of the instrument is 0.5 cm-1. Numerical Method The rigorous form of the one-dimensional Hamiltonian adopted here follows that employed by Meakin, Harris and Hirotais i.e.
H -
Res = 0.5 c m - ' , 7 0 0 Scans Triangular Apodization
I .-0 v) .E
* 0 E
105.0 170.3
250
I
I
230
210
I
The small pseudo-potential term f ( a ) arises from the operation of the momentum operator on the various inertial parameters; it has been neglected in the ensuing calculations. Meakin, et aL3 indicated that the above form of the Hamiltonian is not unique; indeed, other equally good approximations have been employed by other authors.4-6 In molecules such as ethylamine, the internal rotation can be referred to a plane of symmetry (even function of a) and the potential is adequately represented by a rapidly converging Fourier cosine series
1 V(a) = - V,(1 - cos a) 2
1 + SV,(l - qw 2 4 + 1
-V& - cos3Cu)*2 Likewise, the angle dependence of F can be expressed as F ( a ) = Fo F,cosa + F2coL32a F,cos3cu +... The matrix elements of the torsional Hamiltonian were calculated with the use of free rotor functions (exp (kim a)) as basis. Forty-one basis functions were found sufficient to assure convergence of the energy levels of interest. The torsional energies were obtained by diagonalizing the Hamiltonian matrix using the Givens-Householder method. The potential constants which best fit the observed transition frequencies were determined by a NewtonRaphson iterative proceduce. The structural parameters assumed for ethylamine are given in Table I along with the coefficients for the angle dependence of F(a).The latter were evaluated by assuming simple rigid rotation of the amino group about the C-N bond.
+
+
Results and Discussion A . Spectra. In the trans conformation of ethylamine the direction of the nitrogen lone-pa.ir orbital is trans to the C-C bond. The two equivalent gauche forms are achieved through rotation by approximately *120" around the C-N bond. The degeneracy of the vibrational levels of,the two gauche conformations is removed by quantum mechanical tunnelling through the gauche-gauche potential. barrier. The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
1
I
170 i50 Wavenumbers ( c m - ' )
190
I
130
Figure 1. The far-infrared spectrum of CH3CH2ND2 (instrument, Digilab FTS-14).
TABLE I: Assumed Geometry and Kinetic Energy Parameters of Ethylamine
4
where p a is the momentum conjugate to the internal angle a.F(a)is one-half the effective inverse moment of inertia.
Fateley
N-H = 1.014 C-H = 1.093A C-N = 1.4740A C-C = 1.54A
LCNH = 112.2' LHNH = 105.8' All other angles assumed to be tetrahedral Coefficients of F ( a ) terms, cm-1
FQ
CHsCHzNDz CHCDzND2
5.8894 5.7122
F1
F2
0,0733 0.0194 0,0496 0,0188
F8
-0,0006 -0,0003
Thus, two torsion fundamentals are expected for the gauche form, the separation of which depends upon the magnitude of the central barrier. Figure 1 shows the far-infrared spectrum of CH~CHZNDZ. The spectrum of C H ~ C D ~ N DisZvery similar to that of CH3CHzNDz except for slight differences in the frequency positions. A preliminary analysis of the farinfrared spectra of CHsCHzNDz and CH~CDZNDZis aided by a comparison with the corresponding spectrum of C H ~ C H ~ N H ZThe . far-infrared spectrum of gaseous CH&HzNHz was first reported by Tsuboi, Hirakawa, and Tamagake.7 In the region of the C-N torsion, they observed a doublet at 217 and 215 cm-1 which were unequivocally assigned to the gauche form and another peak at 235 cm-1 which was attributed to the trans form. (A more detailed far-infrared spectrum of CH~CHZNHZ has been recorded by the present authors and will be treated in a separate article.) The major features of the spectrum reported by Tsuboi, et al.,' have been verified. On the basis of isotopic shifts, the corresponding C-N torsion fundamentals of CH3CHzNDz will be 185.8 cm-l for the trans and 170.3 cm-1 for the gauche form (Figure 1). A preliminary calculation indicated that a splitting of the gauche fundamental, due to tunnelling, of up to twice the magnitude observed for CHsCHzNHz would not be seen correspondingly in the spectrum of CHsCHzNDz under the resolution (0.5 cm-1) employed in the present work. The magnitude of separation between the pair of peaks at 158.1 and 151.8 cm-1 and their intensities relative to those of the trans and gauche fundamentals mildly suggest that they are transitions originating from the first excited torsional levels of the gauche form. These assignments and the completely analogous case of CH3CDzNDz are summarized in Table II. The absorption peak at 206.0 cm-1 in the spectrum of CH3CHzNDz (Figure 1) has a counterpart at 203.6 cm-l in the spectrum of CH~CDZNDZ. These frequencies are
805
Rotational Isomerism in Ethylamine
m
A
cm-4
vC vN
1
3 -484.60
484.59
O Z t -441.20 11; .-,420.33 11
41 9.96
101-
361.50
0 2 - -328.02 02'011
-322.10 297.26
10-
1 o+
0
-7
+I
TORSIONAL ANGLE
Figure 2.
The potential curve for internal rotation around the ethylamine. The energy levels shown are those for
TABLE 11: Observed and Calculated C-N Torsional Frequencies (cm-I) Assignment
Observeda
0000'
, -
02: 1 1
11-
-43847 -41754
4 8 7 97 48794
417 1 6
101-
363 2 5
0 2- -323 02'-
98 31 8 56
-2 9 4 1 0
253.04 253.03
10' 10-
251.22 251 21
170.30 1 69.92
01; 01
0.01 0.00
UNPERTURBED
Calculated
20 20-
Olt
-1 1 1.46
00'
C-N bond in CH3CHzND2.
cm-1
"CVN
001
00; 00
-
167.71 167.36'
-111.10
-0.35 -0.36 PERTURBED
The effect of kinetic energy coupling on the C-C and C-N torsional levels of CH3CH2ND2. (A-E sublevels of each VCVN level are not indicated.) Figure 3.
CHaCH2ND2 t 0 + 1 g o* + lT g If + 2g 1- + 2 +
185.8 170.3 158.1 151.8
185.8 170.3 158.1 151.8
CHaCD2ND2 to-1
g o* + 1 F gl++2g1-+2+
182.7 168.2 156.8 149.5
182.6b 168.3 156.3 150.6
*
a Frequency accuracy ie within zk0.2 cm-1. Calculated from the potential function derived from the observed frequencies of CHaCHzNDz.
much lower than those that can be calculated for the C-C torsion (-253 cm-1) based on a microwave measurement of the barrier to methyl rotation in ethylamine.8 They cannot be due to the partially deuterated species (i.e., -NHD) as the absorptions underwent no change in relative intensities with repeated deuterations; also, no bands corresponding to the -NHD moiety were observed in the mid-infrared spectra. They conform, however, to the assignment as difference band of the CCN bend with the C-N torsion of the trans form: 390 - 186 = 204 cm-I for CH~CH~ND and Z 385 - 183 = 202 cm-I for CH~CDZNDZ. B. Potential Function. The potential constants derived from the numerical fitting to the amino torsional frequencies of CH~CHZNDZ are VI = -303.2 (f12.6), VZ = 184.4 (f4.1), v3 = 734.7 (f3.1), and v.5 = -19.3 (f1.6) cm-l. The errors given reflect only those of fitting to the frequencies which have estimated uncertainties of f0.2 cm-1 and do not include those arising from the assumed geometry and the inadequacy of the rigid one-dimensional model. The same potential function was used to calculate the corresponding frequencies of CH~CDZNDZ and found to give excellent agreement with the observed values (Table a). The presence of the small value of v6 in the potential may be superfluous6 as it was necessary only to bring the observed and calculated frequencies of CH3CHzND2 into exact agreement. Its inclusion or omission does not significantly affect the potential. The potential curve for the internal rotation of the amino group in ethylamine is plotted in Figure 2. The gauche form is found to be more stable than the trans by 104 cm-l (297 cal/mol). The gauche-gauche and gauche-
-
trans barriers are 535 and 797 cm-l, respectively. The gauche form has an equilibrium dihedral angle of 126". The above results are in qualitative agreement with the ab initio calculations of Radom, et al.2b These authors predicted an energy difference of 182 cm-i (530 cal/mol) between the two forms, the gauche being more stable. In the convention adopted here (a = 0" for the trans), their potential constants for NHz rotation in ethylamine are VI = -311.3, VZ = 73.4, and V3 = 800.9 cm-1. The difference between the above potential and that derived from the spectral data is seen to be largely in the VZ term. C. Coupling with Other Intramolecular Modes. The extent of interaction of the C-N torsion with the other intramolecular modes must be examined in order to establish the reliability of the simple one-dimensional model used in the evaluation of the potential constants. Tsuboi, et al., 7 suggested that the lower C-N torsional frequency of the gauche form of CH~CHZNHZ compared to that of the trans may be due to the interaction of the gauche C-N torsion with the CCN bend. However, appreciable perturbation effects are not likely to arise from the coupling of these two modes as the fundamental levels involved are widely separated. On the other hand, the position of the C-C torsion fundamental presents the possibility of a significant coupling of this mode with the C-N torsion. The barrier to rotation of the CH3 group in C H ~ C H ~ N Hhas Z been estimated from microwave data8 as 1308 52 cm-l. This corresponds to a C-C torsional frequency of -253 cm-I and will be nearly the same as those of the isotopic species examined here. The spectrum of CH3CHzNHz (not presented in this paper) indeed shows some moderately strong Q peaks in the region of the predicted C-C torsion. However, no such absorptions could be detected in the spectra of CH~CHZNDZ and CH~CD~ND even Z at higher vapor pressures. It can be rationalized that the inherently weak bands of the C-C torsion are enhanced through coupling with the C-N torsion in CH3CHzNHz. Deuteration of the amino group lowers the fundamental levels of the C-N torsion sufficiently so as to preclude appreciable interaction with the C-C torsional levels.
*
The Journal of Physical Chemistry, Voi. 78, No. 8, 1974
806
A. S . Manocha, E. C. Tuazon, and W. G .
Fateley
D. Two-Dimensional Model. On t,he assumption that the potential energy coupling is zero, the two-dimensional Hamiltonian in the rigid-rotor approximation may be written as
that the torsional energy levels of the species under consideration are not sensitive to these cross terms. For example, an unlikely high value of 100 cm-I for V’ affects the energy levels only to the same extent as the kinetic energy cross term does. These potential energy cross terms are expected to be small based upon results on molecules with two methyl tops.13 where CY and p are the dihedral angles corresponding to inE. Comparison of Potential Constants in Substituted ternal rotations around the C-N and C-C bonds, respecMethylamines. It is interesting to note the trend by which tively. The first bracketed term is the one-dimensional V3 changes with increasing substitution by methyl groups Hamiltonian for the amino rotation discussed earlier; the on the carbon atom of methylamine: methylamine7.14 second corresponds to the highly symmetric case of CH3 (683.2 cm-1) ethylamine (734.7 cm-l, present work) rotation where Fc is constant and V(p) is simply (%)V3(1tert-butylamine7 (826 cm-I). The difference in the threecos 3p). fold term (AV3) of methylamine and ethylamine may be The term in F’ is the kinetic energy cross term. F’ is regarded as the effect of substitution by one methyl group easily calculated from the molecular geometry9.10 if its and predicts a V3 value of -838 cm-I for tert-butylasmall angle dependence is neglected. The values obtained mine. A similar trend is also observed in the series ethane for CH~CHZNDZ and CH~CDZNDZ are -0.702 and -0.507 propane n e 0 ~ e n t a n e . lWhile ~ a fundamental explacm -I, respectively. nation is lacking at present, the above observations lend A second-order perturbation was carried out with the some credence to the suggestion that the modification of microwave value of 1308 cm-1 for the barrier to methyl the intramolecular energy under substitution of a hydrogroup rotation and the potential constants derived from the observed C-N torsional frequencies of CH~CHZNDZ. gen atom by a methyl group is approximatly additive.16 With this assumption, it is possible to predict the form of The eigenvectors in the free-rotor basis obtained for each the potential curve for internal rotation about the C-N of the two one-dimensional problems were used to calcubond in isopropylamine late the matrix elements of pa and p a . Figure 3 shows the effect of the kinetic energy coupling on the energy levels. 2V(CY) = Reversal of parities (arising from the two equivalent (AVl(l - COS a ) AV& - COS CY) AVd(1 - COS k)} +, gauche forms) are noted for levels with one or more quanta of the C-C torsion. Not shown in the diagram are the {A+ - eos(CY very closely spaced A and E sublevels comprising each level designated by the quantum number U ~ U N .Certain AV,[1 - dos reversals of the normal ordering of the A and E levels in an independent CH3 rotor have also been found in the perturbed case.ll These features of the perturbed energy levels are expected to complicate the interpretation of the microwave spectrum of ethylamine. The first group of terms refers to the effect of substitution The unperturbed levels OO*, OOt, Ol*, O l t , and 02* of a 0-hydrogen atom in methylamine by a methyl group; shown in Figure 3 are those which correspond to the obhere, AV1 and AVZ are simply the values of VI and VZ served frequencies. The true zero-order levels will be highearlier found for ethylamine. The second group of terms er than these by amounts nearly equal to the small shifts corresponds to the substitution by a second methyl group found in going to the perturbed levels of Figure 3. Thus, 2a/3 out of phase from the first. The basic V3 term is that the estimated zero-order frequencies of CH~CHZNDZ are of methylamine. 188.6 cm-1 for the trans 0 1transition, and O* 1* = Substitution of the proper quantities in the preceding 172.3, 1+ 2- = 160.0 and 12+ = 152.8 cm-I for formula yields encouraging results. It predicts a potential the gauche. Since the changes in frequencies are not large curve for isopropylamine in which the trans form (methand in the same direction, the potential constants are not ine proton trans to the nitrogen lone pair) has an energy appreciably affected. The potential constants obtained by minimum which is lower than that of the gauche by apa fit to the estimated zero-order frequencies are VI = proximately 89 cm-I (0.25 kcal/mol). This is in qualita-327.9, VZ = 193.4, V3 = 746.2, and v6 = -16.1 crn-l. tive agreement with a theoretical calculation17 which preThe deviations of the above potential constants from dicted the trans conformation to be more stable than the those evaluated with the assumption of an independent gauche by 0.67 kcal/mol. Calorimetric and spectroscopic rotor may in fact be smaller than the errors arising from data18J9 indicated that the energy difference between the the use of an assumed geometry. two distinct conformations of isopropylamine is quite Rigorously, the two-dimensional Hamiltonian also consmall. Relative intensity measurements of the two C-D tains several small potential energy cross terms whose efstretching bands of (CH&CDNHz in CC14 solution19 profects may be partially additive or compensating to that of vided an estimate of 0.12 kcal/mol for the energy differthe kinetic energy cross term. The data on CH~CHZNDZ ence, with the trans being the more stable form. and C H ~ C D Z N -are D ~ not sufficient to permit their evalu€onclusion ation since their effect on the energy levels of these two isotopic species are parallel and nearly The potential function for internal rotation around the C-N bond in ethylamine has been derived from the farequal. Because of the predominance of the threefold terms infrared data on CH~CHZNDZ and CH3CDzNDz through a in both the amino and methyl group rotations, the symone-dimensional approximation. In these two isotopic metric cross terms V’ sin 3a sin 3p and V” COS 3a COS 30 species, the energy levels of the C-N and C-C torsional may be considered more important than the others by modes are sufficiently well separated such that the COUanalogy with the case of molecules containing two methyl pling of these two modes does not have a large effect on rotors.12 However, second-order perturbation indicated
-
-
-
-
+
+
CY
-
- -
The Journalof Physical Chemistry, Vol. 78, No. 8, 1974
-
+ +)] + + %)]+
Nrnr Studies of Frozen Aqueous Solutions
a07
the zero-order frequencies. As has been implied, the same one-dimensional treatment cannot be reliably used in the interpretation of the spectrum of C H ~ C H Z N HIn~ .the latter, the proximity of the energy levels of the two torsional modes amplifies the effect of coupling. Acknowledgments. This work has been supported in part by funds from the National Science Foundation (GP-22943). The authors gratefully acknowledge the free computer time provided by the Mellon Institute NMR Facility for Biomedical Research (NIH Grant No. RR00292). Thanks are due Drs. P. Meakin and D. 0. Harris for supplying part of the computer programs used in the calculations. References and Notes (1) Part of the thesis submitted by A. S. Manocha to the Graduate Faculty of Carnegie-Melion Unlversity in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) (a) J. P. Lowe, Progr. Phys. Org. Chem., 6, 1 (1968); (b) L. Radom, W. J. Hehre, and J. A. Pople, J. Amer. Chem. Soc., 94, 2371 (1972).
(3) P. Meakin, D. O.'Harris, and E. Hirota, J. Chem. Phys., 51, 3775
119691. (4) J. D. 'Lewis, T. B. Malloy. Jr., T. H. Chao, and J. Laane, J. Mol. Struct., 12,427 (1972). (5) F. Inagaki, i. Harada, and T. Shimanouchi, J. Mol. Spectrosc., 46, 381 (1973). (6) A. S. Manocha, W. G. Fateiey, and T. Shimanouchi, J. Phys. Chem., 77, 1977 (1973). (7) M. Tsuboi, A. Y. Hirakawa, and K. Tamagake, Nippon Kagaku Zasshi, 89, 821 (1968). (8) Y. S. Li, private communication with Durig. et ai., cited in "Vibrational Spectra and Structure," Vol. 1, J. R . Durig, Ed., Marcel Dekker, New York, N. Y., 1972, p 52. (9) J. F. Kiipatrick and K. S. Pitzer, J. Chem. Phys., 17, 1964 (1949). (10) D. R. Herschbach, J. Chem. Phys., 31, 91 (1959). (11) S. S. Butcher and E. 8.Wilson, J. Chem. Phys., 40, 1671 (1964). (12) L. Pierce and M. Hayashi, J. Chem. Phys., 35, 479 (1961). (13) A. Trinkhaus, H. Dreizler, and H. D. Rudolph, Z. Naturtorsch. A, 23, 2123 (1968). (14) D. R. Lide, J. Chem. Phys., 27, 343 (1957). (15) See ref 8 for a compilation of V 3 values. (16) 0. L. Stiefvater and E.8. Wilson, J. Chem. Phys., 50, 5385 (1969). (17) W. A. Lathan, L. Radom, W. J. Hehre, and J. A. Pople, J. Amer. Chem. SOC.,95, 699 (1973). (18) D. W. Scott, J. Chem. Jhermodyn., 3,843 (1971). (19) P. J. Krueger and J. Jan, Can. J. Chem., 48, 3229 (1970).
Nuclear Magnetic Resonance Studies of Frozen Aqueous Solutions' J. E. Ramire&* J. R. Cavanaugh," and J.
M. Purcell
Eastern Regional Research Center,3 Philadelphia, Pennsylvania 797 78 (Received October 75, 7973) Publication costs assisted by the
U.S.Department of Agriculture
The relatively narrow IH nmr absorptions which arise apparently from mobile water molecules within frozen aqueous solutions were studied for frozen solutions of selected acids, bases, salts, amino acids, polypeptides, and proteins. Tbe changes in integrated intensity of the resonances with temperature can be interpreted in terms of the known phase diagrams of some of the systems studied. Observations concerning amino acids, polypeptides, and a protein suggest that the interpretation of the narrow nmr absorptions as representing water of hydration may be inaccurate. In particular, studies of the 1H and I9F resonances of the frozen KF-protein-HZ0 system indicate that the narrow resonances arise from a solute-rich water phase that could best be described as a liquid-like aqueous phase.
A series of investigations4 has been reported on the surprisingly narrow proton resonance observed from frozen aqueous solutions of a variety of macromolecules. The resonances observed could not be assigned either to the protons in the ice lattice or to those belonging to the macromolecule but the intensity was proportional to the amount of protein present. It was suggested that these resonances arose from water molecules associated in such a way with the macromolecules as to be prevented from joining the ice lattice; that is, they were interpreted as arising from "water of hydration." Our own interests in the interaction of amino acids in aqueous solutions5 prompted us to investigate this phenomenon further. We report here the results of that investigation. Experimental Section The nmr spectra were recorded on a Varian Associates DA-60-IL spectrometers equipped with a variable temperature probe and operated at 60 and 56.4 MHz. Relative
integrated intensities were calculated at least in duplicate from four or more recordings taken with alternating upfield and downfield sweep. The values were adjusted according to the attenuation levels as set on the integrator unit. The temperature in the probe was measured directly by means of a thermistor thermometer placed inside an empty nmr sample tube at the location of the transmitter-receiver coils. The probe assembly was fitted with a standard pressure cap through which the leads from the thermistor were led to a resistance bridge for measurement. The temperature could be measured to within 0.5" and remained stable to *loduring the course of the nmr measurements. All compounds studied were of the highest quality obtainable commercially and were used without further purification. Results and Discussion Two Component Systems. In order to provide a background against which to interpret the narrow resonances The Journal of Physical Chemistry, Vol. 78, NO.8, 1974
