J . Phys. Chem. 1987, 91, 909-913
909
Also note that in Table 111 the Gibbs energy change for the ammonia elimination from L-histidine is lower than that for phenylalanine. This is consistent with the increased electronegativity of the nitrogens in the histidine ring structure. In summary, the general argument of Havir and Hanson” is strengthened by a variety of evidence. This hypothesis provides an approximate and qualitative scheme for ordering the thermodynamics of ammonia elimination reactions in aqueous solutions. Industrial Use of Thermodynamic Results. A principal industrial process which utilizes this reaction is the manufacture of L-phenylalanine. This process is accomplished by using an excess of ammonia to drive process B’ to the right:
consideration of thermodynamic datalg for aqueous ionization reactions at 298.15 K. Specifically, the difference between the enthalpies of ionization of acetic and benzoic acids is only 0.84 kJ mol-’ compared to the value of 7.2 kJ mol-’ observed for the ammonia elimination reactions. Also, the difference between the entropies of ionization of acetic and benzoic acids is 13.5 J mol-’ K-I. This value is very close to the difference (18.1 J mol-l K-I) between the corresponding entropy changes for the ammonia elimination reactions given in Table 111. (19) Larson, J. W.; Hepler, L. G. In Solute-Solvent Interactions; Coetzee, J. F., Ritchie, C. D., Eds.; Marcel Dekker: New York, 1969; pp 1-44. (20) Anderson, K. P.; Greenhalgh, W. 0.; Izatt, R. M. Inorg. Chem. 1966,
xtca
+ x a m m = EPhAla - n H H +
(B’) The optimal product yield under a given set of conditions can be calculated from a knowledge of the equilibrium constant (KBf= l/KB). Note that the calculation of KB is dependent upon both the thermodynamic data given in Table I and some information on the nonideality of the solution (see eq 9). Examination of the results of the equilibrium calculations performed herein indicates that the optimal product yield of phenylalanine will increase by going to lower temperatures and low ionic strengths. The effect of pH is seen to be insignificant in the pH region 6.5-7.0. Clearly, kinetic and other factors also need to be considered in the design of any industrial process.
5, 2106. (21) Izatt, R. M.; Wrathall, J. W.; Anderson, K. P. J . Phys. Chem. 1961, 65, 1914. (22) Das, R. C.; Dash, U. N.; Panda, K. N. Thermochim. Acta 1979, 32, 301. (23) Bates, R. G.; Hetzer, H. B. J . Phys. Chem. 1961, 65, 667. (24) Ojelund, G.; Wads& I. Acta Chem. Scand. 1968, 22, 2691. (25) Prosen, E. J.; Kilday, M. V. J . Res. Natl. Bur. Stand., Sect. A 1973, 77, 581. (26) Williams, V. R.; Hiroms, J. M. Biochem. Biophys. Acta 1967, 139, 214. (27) Barker, H. A.; Smith, R. D.; Wilson, M.; Weissbach, H. J . Biol. Chem. 1959, 234, 320. (28) Goldberg, R. N.; Gajewski, E.; Steckler, D. K.; Tewari, Y. B. Biophys. Chem. 1986, 24, 13. (29) Vagelos, P. R.; Earl, J. M.; Stadtman, E. R. J. Biol. Chem. 1959,234, 490. (30) Certain commercial materials and products are identified in this paper to specify adequately the experimental procedures. Such identification does not imply recommendation or endorsement by the National Bureau of Standards.
Acknowledgment. We thank Dr. Michael Meot-ner for his discussions regarding structural interpretations of the measurements. Registry No. E.C. 4.3.1.5, 9024-28-6; L-phenylalanine, 63-91-2.
Network Topology in Simulated Water Robin J. Speedy,* Chemistry Department, Victoria University of Wellington, Wellington, New Zealand
Jeffry D. Madura, and William L. Jorgensen Chemistry Department, Purdue University, West Lafayette, Indiana 47907 (Received: September 4, 1986)
Configurations of 216 water molecules sampled during the course of isobaric-isothermal simulations over the temperature range -25 to 100 OC at 1 atm pressure, using the TIP4P model, are analyzed to study the hydrogen-bond network topology. Results are presented for the total number of polygons of up to seven molecules and for primitive polygons, being those which have no pair of nonadjacent vertices connected by a bridge which is shorter than either of the paths between these vertices within the polygon itself. We introduce the concept of a reference network, in which each molecule is hydrogen bonded to four others, and report on the temperature dependence of its topological characteristics.
Introduction Most discussions of the structure of liquid water emphasize the dominant influence of hydrogen bonds and, particularly since Bernal and Fowler’s seminal work,’ many have suggested that the instantaneous structure of the Iiquid can be modeled by a random hydrogen-bonded network. Thermal excitation of the network may be represented by broken bonds, as in the several “discrete” models, or in terms of intact but variously distorted bonds as in the “continuum” models.* In this work we blur the distinction between the discrete and continuum pictures by defining a reference network in which each ( 1 ) Bernal, J. D.; Fowler, R. H. J . Chem. Phys. 1933, I, 515. (2) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: London, 1969.
0022-3654/87/2091-0909$01.50/0
molecule is hydrogen bonded to exactly four others, as a limiting case of a discrete model. Our analysis takes configurations generated during the course of previously reported NTp simulations of 216 TIP4P water molecules3 at -25, 0, 25, 60, and 100 “C. Hydrogen bonds are defined by a “saturated-energetic” criterion. That is, any pair of molecules is regarded as being hydrogen bonded if their energy of interaction is less than an arbitrary cutoff value VHB but if, as a result, any molecule would have more than four hydrogen bonds only the four strongest bonds are retained. The properties of the hydrogen-bonded network which result can then be extrapolated to the state in which VHB = 0 and every molecule is hydrogen bonded to exactly four others. The topological characteristics of this “reference network” should (3) Jorgensen, W. L.; Madura, J. D. Mol. Phys. 1985, 56, 1381.
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91,No. 4, 1987
correspond to those of the random network or continuum models of water. At very high temperatures the reference network may not correspond closely to the real state of water, in which many hydrogen bonds are undoubtedly broken (for any common sense definition of a hydrogen bond). However, one expects real water to tend toward a well-bonded network as it is cooled, and the topological properties of that network should converge with those of the reference network at sufficiently low temperatures. An important questions which can be addressed by this kind of approach is what is the structure toward which liquid water tends if it is cooled indefinitely without freezing? If the answer is ice 1, for example, then the reference network topology would display an increasing dominance of hexagonal rings of hydrogen-bonded molecules as it is cooled. The network topology is analyzed in terms of polygon statistics. Previous studies“* of polygon statistics in simulated water have measured the concentration of ”non-short-circuited” polygons, as a function of the hydrogen-bond cutoff parameter. In this work we begin by counting all polygons up to heptagons in the reference state and we show that the effect of breaking bonds, by varying VHB, can be simply described. We then introduce the concept of a “primitive” polygon, which is more restrictively defined than Raman and Stillinger’s4 “non-short-circuited” polygon, and examine the temperature dependence of primitive polygons in the reference state. M o n t e Carlo Simulations. The Monte Carlo simulations carried out for pure water with the TIP4P potential function have been described el~ewhere.~ Briefly, 216 molecules, in a cubic cell with periodic boundaries, were simulated under isothermal and isobaric conditions. Up to five million steps were allowed for equilibration and averages were taken over a run of a further two or three million steps. 100 or 200 configurations, each separated by 10 000 or 20000 Monte Carlo steps, were stored on tape and used in the present analysis.
Bond Definition Any pair of molecules whose energy of interaction is less than a cutoff parameter V H B is added to a list of H-bonded pairs. For “weak” values of V H B (Le. V H B > -2 kcal/mol) a significant number of molecules would, as a result of that procedure, be bonded to more than four other molecules, so the list is revised as follows: whenever a molecule is found to have a fifth H-bonded neighbor the weakest of the five bonds is eliminated from the list. (In practice it is necessary to iterate the H-bond finding routine because the elimination of a bond may allow another bond to form to the other molecule involved.) The main reason for limiting the number of H bonds on each molecule to four is that one can then study the properties of the fully bonded reference state where V H B = 0 and in which every molecule has exactly four bonds. The topological properties of this state are independent of VHB. In practice the bond list was initially constructed for V H B = -0.5 kcal/mol, and then the list was revised for stronger bond definitions by eliminating those bonds with energy greater than VHBfrom the initial list. Values of V H B = -0.5, -2, -3, and -4 kcal/mol were chosen. p B us. VHB.The bond probability p B is defined by
PB = N B / 2 N where NB is the number of H bonds and N is the number of molecules. Many topological properties of the network are determined by p B and are otherwise insensitive to the specific properties of particular models or H-bond definitions. However, p B is dependent on both the model used and on the H-bond definition. To facilitate the comparison of our results (4) Rahman, A,; Stillinger, F. H. J . A m . Chem. SOC.1973, 95, 7943. (5) Hirata, F.; Rossky, P. J. J . Chem. Phys. 1981, 7 4 , 6867. (6) Rapaport, D. C. Mol. Phys. 1983, 50, 1 1 5 1 . (7) Speedy, R. J.; Mezei, M. J . Phys. Chem. 1985, 89, 171. (8) Geiger, A,; Mausbach, P.; Schnitker, J. preprint.
Speedy et al. with those of other works it is useful to represent f i as a continuous function of VHB and the temperature T. A satisfactory empiricism for this purpose is with a, = C I ( T - To),a3 = C3T, CI = 2.02 X (kcal mol-’ K)-I, To = 102 K, and C, = 1.4 X (kcal mol-’)-3 K-I. Equation 1 reproduces the measured values of p B to within about 2% over the ranges -25 OC IT I100 OC and -0.5 kcal/mol L V H B L -4 kcal/mol. The volume of supercooled waterg extrapolates toward that of ice or a clathrate crystal in the vicinity of -50 OC and most measures of structural relaxation times in waterlOJl seem to ve T, = -46 OC. The simplest interpretation of diverging as T these observations is that the structure of water is tending to that of some fully bonded low energy network as T - Ts.Il In a fully bonded networkpBis close to unity for weak values of V H B and one expects the limiting slope S, defined by +
to be zero. On that basis, the temperature at which liquid water is tending to a fully bonded state can be estimated by an extrapolation to find where S 0. According to eq 1 S 0 as T + To = 102 K. Since To is well below T, the present analysis offers little support to the appealing notion that the structure of liquid water is rapidly approaching that of some fully connected network when it is cooled toward T,, but since p B L 0.97 at all the temperatures studied in this work when VHB = -0.5 kcal/mol, the possibility of a fully bonded state at -46 OC is not strongly contradicted.
-
-
Counting the Polygons
The algorithm developed for analyzing the network topology in terms of polygon statistics differs significantly from the original method of Rahman and S t i l l i ~ ~ g e This r . ~ section describes the method used for counting the total number of polygons NtJ. For each configuration and for each value of VHB a list is made of the H-bonded neighbors of each molecule. A record is also kept of bonds which cross the walls of the simulated cell into periodic image cells. Each distinct and nonintersecting chain of H-bonded molecules, up to length 7 or 8, is examined for ring formation. If the first and last molecules of the chain are H bonded to each other and form a closed loop, then the chain is counted as a polygon. A polygon may leave the central cell and may include different periodic images of the same molecule in its length, so long as it returns to its origin and forms a closed loop. The test for closed loops involved a check that every bond in the chain which crosses a box wall is balanced by another bond which returns across the same wall. This check ensures that a chain like I-A-B-CI-A-B-CI-Ais not counted as a polygon A-B-C. The method is applicable in principle to chains of any
-
length. It is simpler and computationally more efficient than the method used by Rahman and Stillinger.4 A list of the “names” of the molecules belonging to each polygon is recorded for later use in the primitive polygon analysis. Total Polygons NtJ. The total number of polygons of size j, N,,,in computer-simulated water has not been reported before. It is, however, a topological characteristic of a network. The distribution of NtJ vs. j differs significantly between the various ice polymorphs and clathrate lattices, so its form in water provides a test for structural similarities between water and those crystals. The computer calculations yield NtJ at four values of VHB, which can be extrapolated to obtain reference values flJwhen p B = 1. Consider a particular state in which there are NB H bonds and NtJ ylygons of size j. Let VHB be varied by a very small amount, sufficient to break just one H bond. If the bond energies are ( 9 ) Hare, D. E.; Sorsensen, C. M. J . Cbem. Phys., to be submitted for publication. (10) Lang, E. W.; Liidemann, H.-U. Agnew. Cbem., Int. Ed. Engl. 1982, 21, 315.
( 1 1 ) Cornish. B. D.; Speedy, R. J. J . Pbys. Cbem. 1984, 88, 1888.
The Journal of Physical Chemistry, Vol. 91, No. 4,1987 911
Network Topology in Simulated Water
100 a
b
Figure 3. Illustrating the primitive polygon definition. In Figure 3a there
are two pentagons and one quadrilateral. All three would be counted as non-short-circuited according to Rahman and Stillinger’s definition, but only the quadrilateral is counted as primitive. In Figure 3b there is one heptagon, one hexagon, and one pentagon. The hexagon and the pentagon would be counted as non-short-circuited,but only the pentagon is counted as primitive.
0
p,’
0.5
0
1
Figure 1. The total number of polygons, N,j, in the system of 216 molecules at 0 OC vs. p$’.
200
1 t
7 1
4
-50
0
1
50 T°C IO0
Figure 2. The total number of polygons in the reference state, flj,vs.
temperature. uncorrelated with the size of the polygons to which they belong then the chance that breaking the bond destroys a polygon of size j is just the fraction of the bonds which belong to those polygons,
so ~ N , , / ~ N=BJ”,,/NB
(2)
and integrating eq 2, taking the reference state where pB= 1 and N,, = fl, as one limit, gives Nt,/*,
= Pd
(3)
This simple result is not exact because when VHBis varied it is the weakest bond which breaks first and weaker bonds may be more likely to belong to strained polygons. However, Figure 1 shows that plots of NtJ agains p $ are precisely linear for j 1 5. The slight curvature of the plots for j = 3 and 4 is consistent with the expectation that the weaker bonds are more likely to be associated with the triangles and quadrilaterals in which the 04-0 angles are strained away from the optimal and tetrahedral geometry. The derivation of eq 3 could apply to any topologically defined structures containing j bonds and is not restricted to polygons. The slopes of the lines in Figure 1, or the pB = 1 intercepts, yield f l J ,the total concentration of polygons of size j in the
reference state where pB= 1. The values of fl, are given in Table I and plotted against temperature in Figure 2. Values of N,, at any other value of pB(or of VHB,eq 1) can be obtained from eq 3. Primitive Polygons Npj In viewing the H bond network in ice lh, ice IC, or ice 11, the human faculty for pattern recognition sees hexagonal rings and tends to ignore the presence of a much greater number of larger polygons. However, the results presented above refer to all polygons and the method used there would, if applied to ice 1h for example, find many more octagons and larger polygons than hexagons. Rahman and Stillinger focussed attention on non-short-circuited polygons: “These are polygons with three or more sides no pair of whose vertices are linked by a hydrogen-bond path shorter (in number of bonds, not geometric length) than the minimal path within the polygon i t ~ e l f ” .Their ~ definition has been used in all subsequent studies of polygon statistics in computer-simulated When applied with the commonly used hydrogen-bond definitions the distribution of non-short-circuited polygon peaks at pentagons or hexagons and falls off rather slowly with increasing size. For example, with VHB = -3 kcal/mol the concentration of 11-sided non-short-circuited polygons is about 60% of the concentration of 5 or 6 sided non-short-circuited polygon^.^ In the present work we adopted a stronger prescription for eliminating the larger irrelevant polygons. We define a polygon to be primitive if no nonadjacent pair of its vertices is connected by a path which is shorter (in terms of number of bonds) than either ofthe paths between those vertices within the polygon itself. The definition is equivalent to saying that a polygon is primitive if no adjacent pair of its sides (or three of its vertices) are common to a smaller polygon. Figure 3 illustrates the simplest two cases in which “primitive polygons” are not equivalent to “non-shortcircuited polygons”. In the common ice and clathrate networks (assuming them to be perfect) the concentration of primitive polygons is the same as the concentration of non-short-circuited polygons. However, when applied to simulated water the two concepts lead to different results. The main difference is that the distribution of primitive polygons decreases sharply beyond hexagons such that the concentration of primitive octagons is effectively zero (no more than two primitive octagons were found in our system of 216 molecules). So the complete primitive polygon distribution can be characterized without going beyond seven-membered polygons. The algorithm for counting primitive polygons is simple. Starting with the largest polygons in the list of total polygons, each set of three sequential molecules belonging to each polygon is tested to see if it also belongs to one of the smaller polygons. It is does not then the large polygon is primitive. NpJ us. pB. The object of this section is to describe the way that N p Jvaries with pBso that an extrapolation can be made to p e = 1 to estimate $,,the number of primitive polygons in the reference state. The analysis given makes some simplifying assumptions about the complex statistics of the network topology.
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The Journal of Physical Chemistry, Vol. 91, No. 4, 1987
Speedy et al.
TABLE I: Summary of Properties of the Reference Statea T= T= T= T= T= 60 "C 100°C -25 "C 0 "C 25 O C
%
3 4
5 6 7
7 23 70 42 3
3 4
5 6 7 B,"
8 28
7 23 80 125 212
5 6 7
16 32 65 116 198
68 110 196
8
12
31 4
54 40 4
16 31 54 31 4
20 34 49 30 4
.3 12 128 813
'6 15 132 787
.6 12 131 764
1 21 143 748
30
28 61
.3 11 137 899
4
72 114 208
12 31 67 121 199
20
35
I
I
6
"e1
is the total number of polygons of sizej. T jis the number of primitive polygons of size j . B," is the fitting constant in eq 10 which allows values of NpJ to be estimated for any value of pB (or for any value of VHB, using eq 1).
Of the Ntj polygons of size j , those which share a pair of sides with one or more smaller polygons are nonprimitive and do not contribute to NpJ. Let Bj be the total number of independent bridges which make polygons of size j nonprimitive. The change that a particular one of those Bj bridges affects a particular one of the polygons of size j is l/NtJ and the chance that it does not is 1 - l/N,J. If we assume that the Bj bridges are distributed randomly among the Ntj polygons, then the chance that a particular polygon survives as a primitive polygon is Npj/Nlj = (1 - 1/NtJB]
-50
50 T"C 100
0
Figure 4. The number of primitive polygons in the reference state,
SJ,
vs. temperature.
-13 I
I
-14 ENERGY PER
MOLECULE
(4)
-15
The problem is now reduced to that of describing how Bj varies with pB. Let Nji be the number of pairs of sides which all polygons of size j share with all polygons of size i , and let
-16
R \\
6OoC
1
j- I
Bj = a.CNj, li13
(5)
where the constant aj is introduced to allow for the nonindependence of many such side sharing. (For sample, a pentagon which shares a pair of sides with a triangle must also share two pairs of sides with a quadrilateral, but this would count as only one independent bridge.) Now consider a small change in VHB,just sufficient to eliminate one H bond. The chance that the bond which breaks is one of the 0' + i - 2)Nji bonds of the rings which have two bonds in common is (j i - 2)N,//NB so
+
dNji/dNB = (j + i - 2)Nji/NB
(6)
and, integrating this result, with the reference state as one limit, gives
N., JI = $pBu+f-2)
(7)
The above analysis neglects the Occurrence of rings sharing more than one pair of bonds, for which the exponent (j+ i - 2) would be inappropriate. From eq 5-7
For values of pB> 0.5 the sum in eq 8 can be approximated by its largest term, i = j - 1, so that Bj =
ajqJ-pB(2j-3)
(9)
Combining eq 4 and 9, and taking logarithms, yields In (Npj/Nlj) = B7pB7 In (1 - l/Nlj) where BY = a j y j - , and y = 2 j
- 3.
(10)
-17
' 3
i
4
5
POLYGON SIZE
6
7
Figure 5. The H-bond energy per molecule for molecules belonging to primitive polygons of size j vs j . The H-bond energy per molecule includes only its energy of interaction with the molecules to which it is hydrogen bonded and was calculated for the case VHB= -0.5 kcal/mol when pB> 0.97 and most molecules participate in four H bonds.
The only unknown in eq 10 is the constant B," which represents the number of independent bridges which make polygons of size j in the reference state nonprimitive. The values of B," were determined by regression and are listed in Table I. Values of calculated from eq 10 when PB = 1 are probably accurate to within about 10%. They are shown in Figure 4. Primitive Polygons in the Reference State Figure 4 shows the temperature dependence of the number of primitive polygons in the reference state. Because eq 10 is not exact there are uncertainties of up to 10% in due to the extrapolation to pB = 1 in addition to the statistical errors in the values of NpJ, so the apparently nonmonotonic variations for j = 5 and 6 may not be real. The smaller rings with j = 3 and 4 are tending to disappear as the temperature is reduced and their concentration could be extrapolated to zero at about -80 OC. (The temperature scale is of course a function of the TIP4P potential.) The decrease is expected on the grounds that the 0-0-0 angles in these small rings are necessarily distorted away from the stable tetrahedral geometry. Figure 5 confirms that the molecules belonging to the smaller primitive polygons make weaker H bonds than those
qJ
qJ.
qJ
J . Phys. Chem. 1987, 91, 913-919 belonging to pentagons or hexagons. Primitive heptagonal rings are rare at all temperatures, reflecting the fact that most heptagons are bridged by smaller polygons. The chance that a heptagon survives bridging to remain primitive is only about 2%. In terms of primitive polygons then, the network topology - --_in water is dominated increasingly by pentagons and hexagons as the temperature is reduced. These are the primitive polygons which feature in the low density ices and the common clathrate crystals. They are also the smallest polygons in which the 0-0-0 angles can approach their energetically optimal geometry and, as Figure 5 shows, the molecules belonging to five- and sixmembered primitive polygons make stronger H bonds than those belonging to smaller or larger rings. As noted earlier, one advantage of the primitive polygon concept over the non-short-circuited polygon concept is that the primitive polygon distribution in water can be characterized without going beyond heptagons. Another important difference may be noted by referring to Figure 3a. The two pentagons shown there be counted as being non-short-circuited, but not as being primitive. Because the structure shown includes a quadrilateral, several of
913
the 0-0-0angles involved are necessarily distorted away from the ideal tetrahedral geometry. Thus, the primitive polygon concept may focus more precisely on energetically stable structures in the network.
Conclusions In the previous work it was established that the isobaric-isothermal TIP4P Monte Carlo simulations correctly reproduce the anomalous expansion of water when it is cooled below 4 "C at 1 atm. The present analysis shows that the hydrogen-bonded network topology becomes increasingly dominated by the primitive hexagons which feature in the low density ices and the primitive pentagons which feature in the low density clathrate lattices, as the water is cooled. The water molecules belonging to the pentagons and hexagons are more strongly hydrogen bonded than those belonging to smaller or larger rings. Acknowledgment. Gratitude is expressed to the National Science Foundation for support of this research at Purdue, Registry No. H20, 7732-18-5.
Alkylperoxy and Alkyl Radicals. 3. Infrared Spectra and Ultraviolet Photolysis of I-C3H,02 and i-C,H, Radicals in Argon Oxygen Matrices
+
G. Chettur and A. Snelson* IIT Research Institute, Chicago, Illinois 60616 (Received: April 30, 1986)
Isopropyl radicals, formed by the pyrolysis of bis(1-methylethyl)diazene, were isolated in matrices of Ar + 10% O2and pure Ar. IR spectra of the trapped species were obtained. By use of oxygen isotopic labeling, the i-C3H702radical was identified and a partial vibrational frequency assignment was made. Under Hg arc irradiation, the i-C3H702radical was destroyed. Some of the photolysis products were identified. IR spectral data were obtained for the i-C3H7radical. Under Hg arc irradiation, i-C3H7was converted to n-C3H7,CH4, C2H4, possibly CH3, and some unknown species.
Introduction In the first paper in the series' we reported on the IR spectra of the methylperoxy radical and its dimer, dimethyl tetroxide. As noted in the previous paper, alkylperoxy radicals are important intermediates in the low-temperature oxidation of organic materials. Little is known spectrally about these species. The isopropylperoxy radical, one of the topics of the present paper, is known to have a broad and unspecific absorption band in the UV (220-280-nm r e g i ~ n ) . ~More , ~ recently, a structured electronic spectrum of the isopropylperoxy radical has been identified in the near-infrared showing a well-defined sequence of bands characteristic of an 0-0stretching mode.4 No infrared spectrum has been reported for the species. Alkyl radicals are also important intermediates in combustion processes.s At low temperatures and in the presence of oxygen they react readily to form alkylperoxy radicah6 Recent work has greatly improved the spectral characterization of low molecular weight alkyl radicals. UV spectra of the isopropyl and other alkyl radicals have been reported7 in the 195-370-nm wavelength range. (1) Ase, P.; Bock, W.; Snelson, A. J . Phys. Chem. 1986, 90, 2099. (2) Thomas, J. K. J . Phys. Chem. 1967, 71, 1919. (3) Adachi, H.; Basco, N. Int. J . Chem. Kine?. 1982, 14, 1125. (4) Hunziker, H. E.; Wendt, H. R. J . Chem. Phys. 1976, 64, 3446. (5) Benson, S . W.; Nangia, P.S . Acc. Chem. Res. 1977, 12, 223. (6) Ruiz, R.P.; Bayes, K. D. J . Phys. Chem. 1984, 88, 2592.
0022-3654/87/2091-0913$01 S O / O
Infrared spectra, of varying degrees of completeness, have been reported for the C2 through C, alkyl radicals by using the matrix isolation technique.* As a result of these studies, the general form of primary, secondary, and tertiary Siicyl radical IR spectra has been characterized. In this paper the iriirared spectrum of the isopropylperoxy radical is presented, together with data on its photolysis in the UV. During the study, the IR spectrum of the isopropyl radical was also obtained. Some new data are presented on this radical's vibration frequencies and on its photolysis in the UV.9
Experimental Section The matrix isolation cryostat and the molecular beam pyrolysis tube furnace assembly used in the study have been described previously.10 Isopropyl radicals were produced by the pyrolysis of bis( 1-methy1ethyl)diazene at -300 O C . To form isopropylproxy radicals, isopropyl radicals were allowed to react with argon matrices containing 10% oxygen during the trapping process. Isopropyl radicals were trapped in argon matrices when IR spectra of this species were studied. Matrix deposition times varied from 20 to 70 h. Absolute values of the radical matrix gas dilution ratios (7) Wendt, H. R.; Hunziker, H . E. J . Chem. Phys. 1984, 81, 717. (8) Pacansky, J.; Brown, D. W.; Chang, J. S. J . Phys. Chem. 1981, 85, 2562 and references cited. (9) Pacansky, J.; Coufal, H . J . Chem. Phys. 1980, 72, 3298. (10) Butler, R.; Snelson, A . J . Phys. Chem. 1979, 83, 3243.
0 1987 American Chemical Society
