J. Phys. Chem. 1984, 88, 163-167
163
Deuterium-Protium Separation Factor between Hydrogen and Liquid Methanols J. H. Rolston* and K. L. Gale Physical Chemistry Branch, Atomic Energy of Canada Limited Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ IPO (Received: March 4, 1983; In Final Form: June 6, 1983)
The overall deuterium-protium isotope separation factor, a, between hydrogen and the hydroxyl group of liquid methanols containing both CHI and CD3substituents has been measured directly between -30 and +64 OC. The temperature dependence of the respective separation factors is given by the equations In a(CH3) = -0.4641 + (540.4/T) + (9849.5/T2) and In a(CD3) = -0.8183 + (708.4/T) - (15263/p) where Tis the absolute temperature. The 01 values are about 12% larger than that for liquid water. The equilibrium constants, K1 and Kz, for isotopic exchange between hydrogen and the isotopically substituted methanol vapors were calculated from the a values and the corresponding liquid-vapor separation factors a,. At 25 "C the experimental values of K , and K , differ by 7%, indicating that methyl deuteration exerts a significant secondary isotope effect on deuterium-protium fractionation between methanol vapor and hydrogen. A comparison of equilibrium constants is made with theoretical values calculated from the ratios of partition functions of molecular hydrogen and those derived from the available spectroscopic data of the isotopically substituted methanols.
Introduction Equilibrium isotope separation factors measured between molecular hydrogen and selected molecules can be compared with theoretical estimates calculated from the free energy change associated with the difference in the zero-point energies of the isotopically substituted hydroxyl group minus the corresponding difference for the isotopic hydrogen molecules by using the technique described by Bigeleisen and Mayer' and Urey.z For the HzO-Hz system such a comparison indicated that there was less than 0.5% discrepancy between theory3 and e~periment.~ This agreement decreases somewhat as the molecular complexity of the system increases and is of the order of 1-3% for the NH3-H2 system5 and 4-5% for the methylamineHz system.6 In part, this increased discrepancy can be attributed to the difficulty of obtaining sufficiently precise molecular potential functions to calculate the vibrational frequencies required to evaluate the partition functions. The magnitude of the overall deuterium-protium separation factor, a , between molecular hydrogen and liquid methanol is largely governed by the equilibrium isotope effect on the deuterium distribution between hydrogen and the hydroxyl group of methanol vapor and to a lesser extent on the fractionation effect arising from the different volatilities of the protio- and deuteriomethanols. At low deuterium concentrations the isotope distribution in the gas phase is governed by the equilibrium constant K , for exchange between methanol vapor (v) and hydrogen gas (g) C H ~ O H ( V ) HD(g) C H ~ O D ( V ) H,(g) (1)
+
+
A similar equation can be written for the methyl-deuterated alcohols C D ~ O H ( V ) HD(g) C D ~ O D ( V ) HZ(g) (2)
+
+
N o experimental values of K I have been reported although estimates can be derived from published values of the partition functions. Replacement of a proton of H,O with a methyl substituent is not expected to change the equilibrium constant by more than a few percentage points in view of the recent study on met h ~ l a m i n e - H ~ However, .~ the direction of this change will be reflected in the position of the deuterium distribution between methanol and water and can therefore be used as an independent method of measuring the equilibrium constant for the methanol-water system.' The more subtle change of replacing methyl (1) J. Bigeleisen and M. G. Mayer, J . Chem. Phys., 15, 261 (1947). (2) H. C. Urey, J . Chem. SOC.,562 (1947). (3) R. D. Bardo and M. Wolfsberg, J . Phys. Chem., 80, 1068 (1976). (4) J. H. Rolston, J. den Hartog, and J. P. Butler, J. Phys. Chem., 80, 1064 (1976). ( 5 ) J. Bron and M. Wolfsberg, J . Chem. Phys., 57, 2862 (1972). (6) J. H. Rolston, J. den Hartog, J. P. Butler, L. Silberring, and Hs. H. Giinthard, J . Phys. Chem., 84, 2170 (1980). (7) D. E. Clegg and J. H. Rolston, J . Chem. SOC.,Chem. Commun., 1037 (1978).
0022-3654/84/2088-0163$01.50/0
protons adjacent to the exchange site is expected to be even less pronounced. The ability to precisely determine values of overall separation factors between hydrogen and various molecules with low concentrations of deuterium in the exchanging group has prompted this study. The objectives were to place the value of a for hydrogen and liquid methanol on firm theoretical and experimental ground and to assess the change in a of replacing the three methyl protons of methanol with deuterium atoms. Experimental Section Procedure. The flask used to equilibrate hydrogen and liquid methanol was similar to that described previously! The solid glass barrel of the Young valve was extended with a 4-cm length of tungsten wire. A stainless steel wire basket was attached and several catalyst spheres of a wetproofed platinized carbon catalysts were suspended in the vapor space inside the 50-mL cell volume. In addition, a calibrated precision thermistor (fO.O1 "C, Yellow Springs Instrument Co., Model 4403 1) was cemented into a 2-mm glass tube attached to the top of the cell. The liquid methanols were dried over activated 4A molecular sieves (Linde). Several experiments were done with two concentrations of deuterium in the hydroxyl group of both liquid methanols, C H 3 0 H and CD30H. The catalyst was placed in the basket and the cell evacuated to check for leaks. The cell was opened in a N2-purged glovebag and a weighed amount (3.5 g) of anhydrous methanol containing a known deuterium concentration was spyringed into the cell. The cell was sealed and transferred to a vacuum line, and dissolved air and nitrogen were removed by three successive freezepump-thaw cycles. After the third cycle the liquid was frozen and hydrogen of known deuterium atom fraction 0, = 0.0070) was introduced and sealed into the cell. The cell was immersed in a constant-temperature bath at -30 "C and allowed to reach equilibrium. The liquid methanol was gently agitated by a stirring bar actuated by a slowly rotating magnet suspended outside the exchange cell. Following equilibration (6 h at -30 OC, 1 h at +25 "C) gas samples were withdrawn through the pinhole valve at intervals of 30 min and were expanded through a liquid-nitrogen-cooled trap directly into the Consolidated Electronics CEC21-614 or Vacuum Generators 601 mass spectrometers for isotope analysis. Repetitive samples taken over several hours showed no variation (&OS% or better) with time, indicating that equilibrium had been achieved. Temperatures of the bath were read from a calibrated platinum resistance probe (Stow Laboratories Inc., Model 91 1PL) and the thermistor voltage was noted. Differences between the two ranged from +0.3 OC at -30 OC to 0.05 "C at +60 "C. The thermistor temperatures were accepted (8) J. P. Butler, J. H. Rolston, and W. H. Stevens, "Separation of Hydrogen Isotohes", H. K. Rae, Ed., American Chemical Society, Washington, DC, 1978, ACS Symp. Ser. No. 68, p 93.
Published 1984 American Chemical Society
164 The Journal of Physical Chemistry, Vol. 88, No. 1. 1984 TABLE I: Comparison of Infrared (IR) and Nuclear Magnetic Resonance (NMR) Methods for Analysis of Hydroxylic Deuterium
-___
106xa sample
calcd
IR
NM R
A, CH,OH(OD) B, CH,OH(OD) C, CD,OH(OD)
2 3 283 49 488
23414 t 225 4 9 790 e 800
23 304 45 902
a x = deutcriuni atom fraction.
as accurately reflecting the catalyst temperature within the cell. A given series of measurements spanning a range of temperatures was generally completed within 24 h. No exchange of deuterium with the methyl group was detected under these conditions. Standards. Both mass spectometers were calibrated against hydrogen standards prepared by equilibrating hydrogen of natural isotopic abundance with large quantities of isotopically enriched liquid water (deuterium atom fraction, x = 0.005-0.020) in a gas-liquid contacting column packed with wetproofed platinum catalyst spheres.* Hydrogen was recirculated for several hours with constant column temperature while fresh liquid water was trickled once down the column. The deuterium concentrations of these equilibrated standards were calculated from the known separation factor and the deuterium content of the liquid water was prepared by weight dilution of 99.80 wt % D 2 0 with water of natural isotopic abundance ( x = 0.000146). A standard with an isotopic content similar to that of the sample from the exchange cell was mounted above the spiral trap attached to the inlet of the mass spectrometer. Thus, cell and standard were analyzed alternately under nearly identical conditions of inlet pressure and temperature. An allowance for the H3+contribution to the mass 3 peak was made as described previously4 and spectrometer response factors for HD’ ion relative to Hz+ion were calculated. The Dz+ concentration was calculated from the measured equilibrium quantities of HD+ and H2+ions since its concentration was too low (0.3% of HD+) to measure accurately on the CEC 21 -614 spectrometer. Subsequently direct measurements of D2+ on the Micromass 601 supported this procedure. Isotopic Analysis of the Methanols. Two independent methods were used to confirm the initial deuterium atom fractions, X I , of the hydroxyl groups present in the liquid alcohols used in these experiments. Method A . The characteristic infrared absorption of HOD at 2500 cm-’ was used to monitor the deuterium level present in aqueous solutions containing weighed quantities of the alcohols. A series of water standards was prepared by weight dilution of DzO with water of natural deuterium abundance. One-gram quantities of methanol were accurately weighed and added to 10 mL of each aqueous standard. The optical density of each solution was measured in 0.2-mm CaF, cells on a Perkin-Elmer Model 21 spectrometer. The rapid transfer of deuterium between the hydroxylic groups ensured an equilibrium deuterium distribution so that the optical density difference between the broad peak at 2500 cm-’ and the valley at 2800 cm-’ was representative of the total hydroxylic deuterium concentration. One gram of each unknown solution was similarly diluted and compared against the calibration standards. This method gave good agreement with the atom fraction calculated from the stated isotopic abundance of the CH,OD sample ( x = 0.996, confirmed by N M R ) and the atom fraction present in stock methanol, x = 0.000130 (IR). The method was not directly applicable to the deuterated alcohol due to interference from the CD, absorption and the nonavailability of CD,OH which was known to contain only natural levels of hydroxylic deuterium. A comparison of calculated and measured concentrations is given in Table I together with the results from the N M R method. Method B. Nuclear magnetic resonance proved to be the only method appropriate for determining the deuterium concentration in the hydroxyl group of both the CD3 and CH3 alcohols. A series of alcohol/water mixtures was prepared by adding weighed (1-g) quantities of a series of prepared water standards ( x = 0.25-0.99) into 40 g of methanol. The deuterium spectrum of each was
Rolston and Gale recorded on a Bruker HX-90 spectrometer equipped with a Nicolet 1020A signal averager. Spectra were accumulated by using the hydroxylic proton resonance as the lock line. Comparison of signals from methanol sample A, diluted appropriately, against those from the standards gave a concentration in good agreement with the IR method. The hydroxylic deuterium atom fraction of the CD, alcohol was determined similarly as xi = 0.045902. Calculations. The deuterium atom fraction present in the liquid phase at equilibrium, xe, was calculated for each temperature with the aid of a deuterium mass balance taken over two phases whereby xe =
+ 2 R V - ye) 1 + Sa,-’
~ ‘ (+ 1 S)
(3)
The symbols XI, y’, and y e refer to the initial (i) or equilibrium (e) deuterium atom fractions present in the liquid and hydrogen, respectively. R is the mole ratio of the initial hydrogen to liquid methanol at equilibrium and S is the mole ratio of vapor to liquid at equilibrium. The separation factor, av,between methanol vapor and liquid is defined in terms of the respective atom fractions ze and xe by eq 4. \ a, = x y 1 - ze) / { z y 1 - xe)]
(4)
At the low deuterium concentrations used in this study, cyv, in eq 3, can be equated without significant error to the ratio xe/ze, It follows that xe is essentially equal to x’ plus a small correction which approaches 2.5% when the relative numbers of moles of hydrogen, methanol vapor, and liquid methanol at 25 “ C are 0.03 1:0.0024: 1.OOO,respectively.
Results At low deuterium concentrations the isotopic separation factor, a, between hydrogen and liquid methanol is defined in terms of the atom fractions of deuterium to protium in the liquid methanol, xe, and in the hydrogen gas, ye, a t equilibrium by a = xe(1 - y”//((l - xe)ye]
(5)
The atom fraction of deuterium present in the liquid at equilibrium at each temperature was calculated from eq 3. Since the solubility of hydrogen in liquid methanol is very low over the temperature range studied (mole ratio of dissolved hydrogen to liquid less than 1.5 X neglect of the dissolved hydrogen does not alter the calculated equilibrium concentrations of the liquid methanol at equilibrium by more than lo-,%. Representative results from several runs with initial deuterium fractions of x’ = 0.023304 and x’ = 0.49790 for the CH,OH alcohol are listed in Table 11. (See paragraph at end of the text regarding supplementary material.) Also jncluded are values typical of the less extensive results for the deuterated alcohol. Both complete sets of data are plotted as a function of temperature in Figure 1. The data were fitted to a weighted nonlinear least-squares regression for one dependent variableg to eq 6 and 7, respectively, in which T is the absolute
+ (540.4/T) + (9849.5/p) = -0.8183 + (708.4/T) - (15263/P)
In a (CH,) = -0.4641
(6)
In a (CD,)
(7)
temperature. these are represented by the solid lines in Figure 1. Weights for the dependent variable In a were set equal to a I 2 divided by the variances of the measured a, obtained from the standard deviation of repetitive measurements. Typically the standard deviation varied from 0.03 to 0.06. The standard deviation in In a obtained from the residual variance in the weighted least-squares fitting of eq 6 is 0.01, which corresponds to a 1.O% error in a over this range of temperatures. This is comparable to error estimates of 1.5% and 1.8% obtained with less extensive data bases for the separation factors of the HzO-H2 and CH3NHz-H2 systems. A comparison (Figure 1) of the methanol-H2 separation factors with that of the liquid water-Hz system shows (9) D. M. Himmelblau, “Process Analysis by Statistical Methods”, Wiley, New York, 1970, p 114.
Deuterium-Protium Separation Factor
The Journal of Physical Chemistry, Vol. 88, No. 1. 1984 165 TABLE 11: Representativea Separation Factors, a , between Hydrogen and Liquid Methanols .II___
I -
deuterium atom fractions
data setb
I
temp,
I I1
111
6.5-
U
0
6.0-
IV
I-
o
2 0
V
5.5-
I-
a
E
2
1o6ye
1O6xe
a
-20.22 -20.25 -20.14 -20.14 -20.14 -20.14 26.55 26.55 26.50 26.50
CH,OH-CH,OD 3801 3197 825 1 8335 8231 8314 5519 5550 5538 5542
23486 23481 49410 49404 49412 49406 23084 23083 23083 23083
6.303 6.310 6.241 6.184 6.263 6.199 4.258 4.234 4.243 4.240
-19.94 -19.94 -19.85 -19.85 25.60 26.00 25.85 25.85
CD, OH-CD, OD 5858 32711 5882 32114 5363 32116 5901 32111 11651 45 365 11873 45 355 11713 45362 11725 45 362
5.151 5.721 5.146 5.708 4.031 3.954 4.009 4.005
OC
a See end of text regarding supplementary material.
5.0-
Data for
I, 11, and IV were obtained on Micromass 601, and those for 111 and V with the CEC-21-614 residual gas analyzer. 4.5 -
35 1 4.0-
3.5-
1
3.0-40
-20
0
20
40
60
I
TEMPERATURE, O C Figure 1. Temperature dependence of deuterium-protium separation factors between molecular hydrogen and liquid water (dashed line) from ref 6, liquid CH,OH (top line), and liquid C D 3 0 H (middle line): XI = 0.023414 with CEC 21-614 spectrometer (0)and with MM-601 (e);x' = 0.049790 with MM-601 (X); x' = 0.045902 with CEC 21-614 (0); XI = 0.031987 with MM-601 (E).
it to be significantly larger (+14.5% at 5 OC and +11.5% at +60 "C). A similar enhancemht was previously noted for CH3NH2-H2 relative to the NH3-H, system.6 Values of the gas-phase equilibrium constants K , and K2 were obtained from the measured overall separation factors a between the liquid methanols and hydrogen. When exchange is limited to hydrogen gas and the hydroxyl group of the alcohol, it follows that a t low deuterium concentrations K 1 = a(CH3)/2a, where a, is the vapor-liquid fractionation factor for the CH30H-CH,0D system. A corresponding expression can be written for K2. The vapor-liquid fractionation factors a, can be equated to the vapor pressure ratio of the protio-hydroxyl to deuterio-hydroxyl methanols for both the CH,OH/OD and CD,OH/OD systems. The equations of Jansco and Van Hooklo were used to calculate the vapor pressure ratios required t o convert the measured separation factors, a , into equilibrium constants K l and K2 for comparison with the theoretical estimates derived from statistical thermodynamics. These were fitted by a least-squares regression to eq (10) G. Jansco, and W. A. Van Hook, Chem. Rev., 74, 689 (1974).
10-1
1
-40
-20
1
0
20 TEMPERATURE
1
1
40
60
,OC
Figure 2. Comparison of experimental and theoretical gas-phase equilibrium constants for deuterium-protium exchange between hydrogen and methanol vapors: Lines I (CH30H) and I1 (CD30H) obtained from eq 6 and 7 with appropriate corrections for vapor-liquid fractionation effects; dashed line 111, the common curve obtained with methanol frequencies of SMG, ref 11. The 99% confidence limits for predicted mean values from eq 8 and 9 are shown as error bars at -30, 25, and +60 OC.
8 and 9, which are plotted as a function of temperature as curves I and I1 in Figure 2.
In K1 = -1.1056
+ (540.0JT)+ ( 1 1 0 9 / p )
(8)
In K2 = -1.3204
+ (613.8/T)
(9)
- (8782/p)
The curve I11 (dashed line) lying between curves I and I1 represents the common curve obtained for both the CH, and CD3
Rolston and Gale
166 The Journal of Physical Chemistry, Vol. 88, No. 1, 1984 TABLE 111: Comparison of Gas-Phase Methanol-Hydrogen Equilibrium Constants at 25 OC equilibrium constants
origin exptl (this work)a anharmonic (i) SMG set 11, ref 11 (ii) Mallinson, ref 13 (iii) SWB ab initio, ref 14
harmonic (i) SMG set I1 (ii) Mallinson (iii) SWB ab initio
K,(CH,OH/HD)
(CD,OH/HD)
2.05 i: 0.006
1.90 i 0.006
1.99
1.99 1.85 2.02
1.85
1.95 2.21
2.09 2.24
K2-
2.26 2.08 2.24
a Error limits represent 99% confidence intervals for mean values predicted from eq 8 and 9.
alcohols using partition functions calculated from the fundamental anharmonic frequencies determined by Serrallach, Meyer, and Giinthard (SMG)'I and anharmonically corrected H2/HD partition functions.l2
Discussion Ideal gas phase partition function ratios can be used to calculate a theoretical estimate of the equilibrium constants of reactions 1 and 2. According to Urey2 the equilibrium constant of the general deuterium exchange reaction A W g ) + D(g) AD(g) + H(g) (10) in which A H = C H 3 0 H , CD,OH, or Hz can be written as KAH = qADqH/(qAHqD) (11) in which qAH is the molecular partition function of AH. Equilibrium constants of reactions 1 and 2 can then be expressed as ratios of the appropriate KAH pairs. The KAH functions are equal to the F values used by Bigeleisen and Mayer' and it follows that
-
in which
uAH
is the symmetry number of A H and UAH,= ~ C W A H , / ~ B T
(13)
where oAH, is the ith vibrational frequency of AH. The product is taken over the 3n - 6 vibrational frequencies for n-atom, nonlinear molecules. Values of KAH for the C H 3 0 H / O D and CD,OH/OD pairs at 25 'C were calculated from the fundamental frequencies, reported by SMG" as 13.576 and 13.560, respectively. The corresponding value for the Hz/HD pair was evaluated from the anharmonically corrected expression of Bron, Chang, and Wolfsberg (BCW)lZ as K" = 6.813. These lead to values of K 1 = 1.993 and K2 = 1.990, indicating that methyl deuteration has no significant effect on the equilibrium constant. Comparison with the experimental data in Figure 2 shows that the theoretical curve lies between the two experimental ones. At 25 'C the experimental value for C H 3 0 H is 4% above the theoretical estimate while that for C D 3 0 H is 3.5% below as shown in Table 111. However, less satisfactory agreement is obtained if the deharmonized frequencies, from a 28-parameter harmonic force field reported by Mallinson,13 are used to evaluate the KAH terms for the methanol pairs as in this instance K 1 = K2 and both lie below the experimental values (-9.8% and -2.8%, respectively). While both sets of calculated frequencies are in reasonable agreement with the observed fundamentals, their use in the direct evaluation of KAH for the methanol pairs is unsettling. Use of the anharmonic frequencies (1 1) A. Serrallach, R. Meyer, and Hs. H. Giinthard, J . Mol. Specfrosc., 52, 94 (1974). (12) J. Bron, C. F. Chang, and M. Wolfsberg, Z . Naturforsch. A , 28, 129 (1973). . (13) P. D. Mallinson, J . Mol. Spectrosc., 58, 194 (1975).
calculated by Schlegel, Wolfe, and Bernardi (SWB)14 from an ab initio study using an anharmonic force field gives values of K , and K 2 close to those obtained with the SMG frequencies. However, in this instance the theoretical value of K2 is greater than K 1 ,which is contrary to experiment. Similar difficulties in estimating isotopic equilibrium constants from anharmonic frequencies have been reported by Khurma and Fenby.Is Indeed, it has been notedI6 that it is often difficult to determine small isotopic effects on spectroscopic data with great accuracy and hence use of harmonic frequencies evaluated from a molecular potential function common to the isotopic pair is generally preferred for evaluating partition functions. An attempt to estimate the harmonic frequencies, w, from the anharmonic frequencies, u, given by both SMG and Mallinson has been madel5 by applying Dennison's rule in which w = v / ( l - x ) . Mallinson and McKeanI7 suggest x factors of 0.04 for O H and C H stretching, 0.02 for O H and C H bending, and 0.01 for C-0 stretching frequencies. Factors for the corresponding deuterated species were calculated from the relation xD = x H ( u D / u H ) . Values of KAH for the methanol pairs calculated from these harmonized values were substantially (20%) greater than those which included anharmonicity corrections. Corresponding values of K", for the hydrogen pair calculated under the harmonic approximation, were estimated by applying the correction factor given by BCW for anharmonicity, quantum-mechanical rotation, and deviations from the equilbrium moments of inertia. These effects combine to increase K" from 6.831 to 7.192. This value agrees reasonably well with K" = 7.099 calculated by Bigeleisen and Ishida'* at 300 K. Values of the equilibrium constants calculated from these harmonized KAH values are listed in Table 111 for the two sets of vibrational frequencies. Deletion of the anharmonicity corrections raises the calculated equilibrium constants by about 15%, indicating that the anharmonicity effect is substantial and that it arises primarily from the methanol partition function. The equilibrium constants K , and K2 from the harmonized SMB frequencies are 11% and 19% above the experimental values while those from the Mallinson frequencies are greater by 2% and 9%, respectively. Values of the equilibrium constants similar to those from the harmonized SMG frequencies were obtained directly from the harmonic frequencies given by SWB. The overall agreement between experiment and theory is poor. All three estimates based on harmonic frequencies suggest there should be no change in the equilibrium constant arising from deuteration of the methyl substituent. This again is contrary to the observed reduction of 7.3% noted in Table 111. Although the use of harmonic frequencies is recommended,16 it has been necessary to apply Dennison's rule and a somewhat arbitrary set of x values to harmonize the frequencies of SMG and Mallinson. This has been questioned by SWB.14 Large anharmonicity effects seem inconsistent with those reported for partition functions of the isotopic water specieslg when the isotope-dependent constant, Go, is included in the vibrational energy expression for the isotopic molecules. Neglect of Go has been shownZoto lead to erroneously large anharmonicity corrections on partition functions. Thus, the procedureI5 used to estimate harmonic frequencies for a molecule of the complexity of methanol may not be entirely satisfactory. The consistency of the experimental data reported here has been checked by calculating the equilibrium constant for deuterium exchange between methanol and water (eq 14). Several conHDO(g) CH30H(g) + CH30D(g) + H,O(g) (14) flicting values have been reported as noted previously.' At 25 'C a value of K I 4 = 0.58 f 0.02 is obtained by combining the
+
(14) H. B. Schlegel, S. Wolfe, and F. Bernardi, J . Chem. Phys., 67, 4181 (1977). (15) J. R. Khurma and D. V . Fenby, Aust. J . Chem., 32, 465 (1979). (16) M. Wolfsberg, Ace. Chem. Res., 5 , 225 (1972). (17) P D. Mallinson and D. C. KcKean, Spectrochim. Acta, Part A , 30, 1133 (1974). (18) J. Bigeleisen and T. Ishida, J . Phys. Chem., 48, 1311 (1968). (19) M. Wolfsberg, J . Chem. Phys., 70, 5322 (1979). (20) M. Wolfsberg, Adu. Chem. Ser., No. 89, 158 (1969).
J. Phys. Chem. 1984, 88, 167-170
167
measured methanol-hydrogen equilibrium constant (eq 8) with that previously reported for exchange between water and hyd r ~ g e n .This ~ value is consistent with the thermochemical measurement of Bertrand and BurchfieldZ1and the statistically weighted spectroscopic data of Kwart and co-workers.22 It provides evidence that deuterium favors slightly the hydroxyl group of methanol over that of water. A similar conclusion was reached by Khurma and Fenby in a recent reassessmentL5of partition functions evaluated from the deharmonized methanol frequencies of Mallinson and those of Papousek and Pliva (PP) for water.z3 This agreement between experiment and theory for equilibrium constants governing deuterium fractionation in the methanol-water system seems fortuitous' in that the harmonic frequencies of PPZ3 give rise to a Km value for water 10% greater than that calculated with anharmonicity corrections by BCW.I2 A similar overestimate on the KAHfor methanol based on the estimated harmonic frequencies of Mallinson and SWBI4 would tend to cancel in the calculation of KI4for methanol-water but would appear as a correspondingly large overestimate of K , in the methanol-hydrogen equilibrium studied here. The anharmonically corrected BCWI2 partition functions provide equilibrium constants for the hydrogen-water system which are only 2% below the experimental value^.^ This small discrepancy is largely removed when the arising out of deviations multiplicative correction factor, KBOELE, from the Born-Oppenheimer approximation is i n c l ~ d e d Recent .~ experimental evidence in support of this correction term at high deuterium levels has also been obtained.z4 Regrettably there are no comparable partition functions currently available for molecules as complex as methanol. The experimental evidence presented here shows that the equilibrium constant for C D 3 0 H is less than that for C H 3 0 H .
In light of the poor agreement with experiment and the inconsistencies between the theoretical estimates presented in Table 111, it is clear that a close theoretical study of methanol partition functions is desirable. At present the anharmonically corrected partition function ratio of BCW for the hydrogen pair together with those obtained from the fundamental frequencies of SMG or from the ab initio calculations of SWB for methanol give calculated equilibrium constants in reasonable agreement with experiment. However, the observation of a small but measurable secondary equilibrium isotope effect on the deuterium-protium separation factors and equilibrium constants, based upon the predicted mean values at the 99% confidence limit, suggests that the methanol force fields used in the spectroscopic analysis may require further refinements. Some coupling between the hydrogens of the methyl and hydroxyl groups through the torsional frequency is to be expected. At present very few experimental measurements of the effect of methyl deuteration on the torsional frequencies exist. Thus, it is difficult to reach a firm conclusion on the adequacy of the available force fields to predict torsional frequencies and to assess the effect of their coupling to the higher methyl and hydroxyl frequencies which dominate the partition function ratios used in evaluating K , and K 2 .
(21) G. L. Bertrand and T. E. Burchfield, J . Phys. Chem., 79, 1547 (1975). (22) H. Kwart, L. P. Kuhn, and E. L. Bannister, J . Am. Chem. Sot., 76, 5998 (1954). (23) D. Papousek and J. Pliva, Collect. Czech, Chem. Commun., 29, 1973 (1964). ' (24) J. H. Rolston and K. L. Gale, J . Phys. Chem., 86, 2494 (1982).
Supplementary Material Available: The complete Table I1 contains the 64 and 179 experimental values of the overall separation factor and the 'Onstants K2 and K l for the C D 3 0 H and CH,OH alcohols displayed in Figure 1 (6 pages). Ordering information is given on any current masthead page.
Acknowledgment. We thank W. M. Thurston and M. W. D. James for assistance in maintaining the spectrometers and J. D. Halliday for the N M R analyses. Our thanks go to Prof. W. A. Van Hook for communicating his interest in advance of publication. Registry No. CH,OH, 67-56-1;CD,OH, 1849-29-2;HD, 13983-20-5; H2, 1333-74-0;deuterium, 7782-39-0.
Photoionization of Dibenzocarbazoles in Solid Solution. Lowering of the Photoionization Threshold through Hydrogen Bonding with Pyridine M. M. Martin,+*D. Grand,* Laboratoire de Physico- Chimie des Rayonnements, LA75, Bdt. 350, UniversitP Paris-sud, 91 405 Orsay, France
N. Ikeda,* T. Okada, and N. Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: March 22, 1983) A study of the hydrogen-bonding effect on the photoionization energy threshold of 13H-dibenzocarbazole (13DBC) and 7H-dibenzocarbazole (7DBC) in a solid matrix of tetramethylsilane (Me4Si) at 77 K is reported. The charge-separation
process was induced through a two-photon stepwise process by using two different light beams, and followed by recording the delayed luminescence which results from the stimulated electron-cation recombination. The hydrogen-bonding effect was studied by comparing the photoionization efficiency curves obtained in neat Me4Si with those obtained for samples containing 0.4 M pyridine, a concentration at which the DBC molecules are totally hydrogen bonded to pyridine. In the presence of pyridine, the ionization threshold was found to be 1.1 eV lower than that in neat Me$ for bothDBC. For N-ethylcarbazole, which cannot form a hydrogen bond with pyridine, the presence of pyridine was shown to lower the ionization threshold by 0.3 eV. This lowering was attributed to the increase of the matrix polarity when pyridine is mixed with cyclohexane. When this solvent effect was taken into account, hydrogen-bonding interaction with pyridine was shown to decrease the photoionization threshold of both DBC by 0.8 eV. Introduction In a previous study,' we provided direct evidence that a rapid charge transfer occurs from the fluorescent state of carbazole
'
Laboratoire de Photophysique Molkulaire du CNRS, Blt. 213, UniversitB Paris-sub, 91405 Orsay, France. *Present address: Institute for Molecular Science, 38 Nishigonaka Myodaiji, Okazaki 444, Japan.
0022-3654/84/2088-0167$01.50/0
derivatives through hydrogen bonding with pyridine. As a matter of fact, by means of a picosecond photolysis technique, we clearly observed the formation of a charge-transfer state D+-H- --A- f;om the excited singlet state SI of the hydrogen-bonded complex (D(1) M. M. Martin, N. Ideda, T. Odada, and N. Mataga, J . Phys. Chem., 86, 4148 (1982).
0 1984 American Chemical Society
