J . Phys. Chem. 1985,89, 3547-3552 from oxygen lone pair *-bonding to vacant or partially filled d and p orbitals on the metal. The availability of vacant d and p metal orbitals will not lead to additional *-bonding for the hydrogen-metal Q bonds. The peaking of the calculated H-MOH bond energy a t iron may be reflecting d-orbital splitting effects on the bonding. The hydride ligand splitting of the d-orbitals is expected to result in a peaking of the H-MOH bond energy due to the need to fill destabilized d-orbitals for the heavier transition metals. Finally, it is interesting to note the rise in frequency of the MOH bending mode for the H M O H species with increasing atomic weight. A rise in frequency may be expected with increasing atomic weight of the metal due to contraction in size of the metal bonding orbitals which results in a smaller internuclear HM-OH bond distance and increased repulsion between oxygen lone pairs and filled metal d-orbitals. As earlier noted, the zinc adduct reacted only in experiments with photolysis during cocondensation where it formed zinc monohydroxide. Copper monohydroxide was also preferentially formed in experiments with simultaneous photolysis. It is surprising that the copper and zinc monohydroxides are formed since both have unfavorable heats of formation as seen from Table I. The production of MOH might be rationalized if it is assumed that photolysis of the adduct forms an excited-state insertion species HM*OH which can lose a hydrogen atom to form MOH. If this happens in a matrix cage, the hydrogen atom will back react to re-form the adduct. Photolysis during cocondensation on the matrix surface boundary layer would produce the same HMOH* intermediate. However, because the reaction zone of the matrix is soft, decomposition to M O H and H can occur. The formation of ZnOH is interesting in view of the gas-phase abstraction processes which have been observed for Zn, Cd, and H g metal atomsI4 where it was noted that excited-state metal atoms abstracted hydrogen atoms from alkanes. The following equations summarize observed reaction pathways for the various metals with water in argon matrices at 15 K: M = Cr, Mn, Fe, Co, Cu, Zn
3541
M
Ar
+ H2O
M.OH2
M = Cr, Mn, Fe, Co, Cu M-OHI
hu
HMOH
M = Cr, Mn, Fe, Cu, Zn M2
+ H20 x M2.OH2 Ar
M = Mn; x 2 3 M,
+ H2O x M,*OH2 Ar
M = Mn, Fe
2M
+ H20
HMOMH
M=Cr;x21 M
+ 2H2O
hv
H,M(OH)2
+ (2 - x)H
M = Fe
Acknowledgment. This work was supported by the National Science Foundation and the Robert A. Welch Foundation. Registry No. HCrOH, 96759-87-4; HCr(OH)2,96759-88-5; HCuOH, 96759-89-6;CuOH, 12125-21-2;HCoOH, 81514-94-5;HMnOH, 81514-93-4; HMn20H, 96759-90-9; HMnOMnH, 96759-91-0;HFeOH, 81533-93-9; Fe(OH)2, 18624-44-7; HFeOFeH, 96759-92-1; HFe20H, 96759-93-2; ZnOH, 3601 1-55-9;HzO, 7732-18-5;H2I80,14314-42-2; DzO,7789-20-0; HDO, 14940-63-7; Cr, 7440-47-3; Mn, 7439-96-5; Fe, 7439-89-6; Co, 7440-48-4;Cu,7440-50-8;Zn, 7440-66-6;Ni, 7440-02-0.
Reactions of Atomlc Scandium, Tltanlum, and Vanadlum with Molecular Water at 15 K J. W. Kauffman, R. H. Hauge,* and J. L. Margrave Department of Chemistry and Rice Quantum Institute, Rice University, Houston,. Texas 77251 (Received: January 31, 1985)
Scandium, titanium, and vanadium metal atoms were cocondensed with water molecules in an argon matrix at 15 K. The atomic metals were observed to insert spontaneously into the OH bond of water to form the HMOH molecule, which was found to be nonlinear in all cases. Photolysis of the HMOH species produced only the metal monoxide, MD, with the exception of DScOD, which formed ScOD in addition to ScO. Tentative assignments have been made for the H&(OH),, H,Ti(OH),, Ti(OH),, and H,V(OH)2 species. Infrared frequencies and suggested vibrational mode assignments for these molecules are given.
Introduction Matrix isolation a t 15 K provides a convenient method for the study of the chemistry of scandium, titanium, and vanadium atoms with water. The reactions of these early transition metals with moleculat water are expected to be highly exothermic as seen from Table I. Estimates of bond energies were made from comparison with known metal fluoride, chloride, and hydride bond energies.’S2 However, significant exothermicity has not always proven to be (1) Jackson, D.D.‘Thermodynamics of the Gaseous Hydroxides”; Lawrence Livemore Laboratory: Livermore, CA, 1971;UCRL-51137 (2) Gaydon, A. G. ‘Dissociation Energies and Spectra of Diatomic Molecules”; Chapman and Hall: London, 1968.
0022-3654/S5/2089-3547%01 S O f 0
an indicator of spontaneous reaction of metal atoms with water in matrices at 15 K. Previous studies of metal atom reactions have shown that in many cases a metal atom-water adduct is formed without further reaction.)” Gas-phase reaction studies of atomic scandium with water by Liu and Parsod’ have demonstrated that (3) Kauffman, J. W.;Hauge, R. H.; Margrave, J. L. J . Phys. khem., preceding article in this issue. (4) Hauge, R. H.; Kauffman, J. W.; Margrave, J. L. High Temp. Sci. 1984,18, 97. (5)Hauge, R. H.; Meier, P. F.; Margrave, J. L. Eer. Eunsenges. Phys. Chem. 1979,82, 102;J. Am. Chem. SOC.1978,100, 2108. (6)Hauge, R. H.; Kauffman, J. W.; Fredin, L.; Margrave, J. L. ACS Symp. Ser. 1982,No. 179,355-362, 363-376.
0 1985 American Chemical Society
3548
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
Kauffman et al.
TABLE I: Estimated Heats for Metal Atom plus Water Reactions" (kJ/mol)
-
SC+ H20 SC+ H2O SC+ H20 SC+ 2H20 Ti + H20 Ti + H 2 0 Ti + 2Hz0 V + H20 V + H2O
SC t H20 (574
AEpb
HSCOH SCOH + He SCO+ H2 HSC(OH)~ + He HTiOH Ti0 + H2 H2Ti(OH)2 HVOH VO + H2 V + 2H2O H,V(OH),
-230 -2 1 -184 -251 -209 -171 -418 -1 97 -154 -396
+
-+
+
+
-+
+
SctH 0 (1203 C
a
"At low temperatures, AG," = AH," since TAS, 0 and can be neglected. bThe values for the reactions other than those leading to the monoxides are estimated to be accurate only to f80 kJ primarily due to lack of information on metal-hydride bond strengths. The monoxide heats of reaction are derived from experimental values.
SC H20 1220 p )
TABLE 11: Observed and Calculated Frequencies (cm-I) for HMOH Species
M-H stretch (obsd) HScOH, a H2I60 H2I80
1485.1 1485.0 1070.0
D2O
M-O stretch obsd calcd"
jc + H20 :I20 p ) Jfter ifrod.
715.8 694.4 698.2
687.1 699.0
699.7 615.5 697.3
671.9 683.3
HTiOH, a H2I60 H21s0
1538.9 1538.5 1107.7
D2160
H2I60 H2I80
D2I60
M-H obsd
HVOH, a M-O obsd calcd
1583.0 1583.0 1140.3
703.3 678.3 696.6
674.7 686.1
HMO or MOH bend 414.5 412.5
"Calculations assume a diatomic model with the HM and OH groups treated as single masses. water spontaneously reacts to form ScO. Titanium and vanadium have also been shown to cause deoxygenation of epoxides.* This work provides new insights into likely reaction intermediates for the above studies as well as suggesting other oxidative insertion pathways for these highly reactive metals.
Experimental Section The scandium, titanium, and vanadium metals used were commercial samples with a stated purity of +99% with respect to other metals. Any surface oxide coating was mechanically removed from each sample. A molybdenum crucible, 0.25-in. in diameter and 2.5-in. long, was used in a tantalum furnace to vaporize scandium over the temperature range of 1180-1295 O C . A densified carbon crucible with similar dimensions was used to vaporize titanium and vanadium over the temperature ranges 1590-1793 and 1850-2000 OC,respectively. Matheson argon gas (99.99%) was used as the matrix material and was further purified by passing it through a liquid nitrogen trap. H2l60,H2I80(95%), and D20(98.8%) were all degassed with a number of freeze-thaw cycles prior to use. The matrix was photolyzed for 15 min with a 100-W medium-pressure mercury lamp and long-pass Corning filters subsequent to deposition. In some experiments photolysis was carried out during cocondensation. A more complete description of the matrix isolation apparatus, the cocondensation procedure, and photolysis experiment has been given in ref 9-1 1. (7) Liu, K. Parson, J. M . J . Chem. Phys. 1978, 68, 1794. (8) Gladysz, J.; Fulcher, J.; Togashi, S . J. Org. Chem. 1976, 41, 3647. (9) Hauge, R. H.; Kauffman, J. W.; Margrave, J. L. J . Am. Chem. SOC., 1980, 102.
ARGON MATRIX
I
I
I600
I350
1 I100
cm-1
I 8 50
+
Figure 1. Infrared spectra of Sc H20in an argon matrix with a low metal concentration. (A) Low water concentration, (B) medium water concentration, ( C ) high water concentration, and (D) matrix B after photolysis with 300-400-nm light. TABLE III: Observed and Calculated Frequencies (cm-I) for MO Species ScO, b VO, b
H2I60
gas
obsd
965
954.6 914.8 954.8
H2I80 D20 46Ti obsd
4773
obsd
calcd
gas 1002
914.6
obsd
calcd
1029.7 986.4
985.2
1029.3
TiO, b 48Ti gas obsd calcd
49Ti obsd
?i
obsd
H2I60 1015.8 1013.0 1000 1010.5 1007.7 1005.2 H2180 974.6 971.7 969.4 967.5 966.1 963.6 D20 1009.8
Scandium. Scandium metal atoms reacted immediately with water upon cocondensation in excess argon at 15 K. This was evidenced by the appearance of the "a" product bands in Figure 1A,B immediately after cocondensation. The product bands were present at low metal and water concentrations, indicating the a product results from a one metal atom and one water molecule reaction. The a band at 1485.1 cm-I is assigned to the Sc-H stretching mode since it undergoes a large deuterium shift of 415 cm-I. The 715.8-cm-' band underwent oxygen-18 and deuterium shifts of 21 and 17 cm-I, respectively (see Figure 3), and is assigned to the Sc-OH stretching mode. The a frequencies are assigned to the HScOH species and are listed in Table 11. (10) Ismail, 2. K.; Fredin, L.; Hauge, R. H.; Margrave, J. L. J. Chem. Phys. 1982, 77, 1617, 1626. ( 1 1 ) Kauffman, J. W. Ph.D. Thesis, Rice University, 1981.
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3549
Reactions of Sc, Ti, and V TABLE I V Observed Frequencies for H,M(OH), Species
H2160 H2160/H2180 D20
HSc(OH)2, c M-H
M-OH
1602.0 1600.6 1176.1, 1174.2
730.1 723.4,' 710.5 714.5
H,Ti(OH),, c M-H
H2160 1688.9, 1665.9 H2I80/H2I60 1686.3, 1665.9 D2160/H2160 1206.0
M-OH
before photolysls
806.0, 775.9' 799.9,', 783.2, 758.9,' 745.7' 800.5,' 788,' 779.9, 763.8," 753.7,' 746"
B
H,V(OH),, c
H2I60 H2180 D2160 a
M-H
M-OH
1601.9 1600.8 1183
787.6 (780.7 side band), 678.3 778.8,' 766.1 (759.9 side band), 670.4 779.0,' 766.0
after photolysis SC + D20
Figure 2. Expanded infrared spectra of Sc + D20 in an argon matrix (A) before photolysis and (B) after photolysis with X = 300-400 nm
Isotopically mixed species.
TABLE V Observed Frequencies for Various Species Resulting from Metal-Water Reactions
light.
ScOD, d M-O stretch D20
699.2
Ti(OH),, g ( x = 3 or 4) 46Ti 47Ti 48Ti
H2I60
850.5
846.8
H2180/H2160 D2l60/H2I60
843.5 820.8; 835.6' 815.1, 824.0,' 831.9,' 839.3'
TiH,, h M-H stretch H2160 annealed D2I60
1647.7; 1611.5 1601.3 1184.9; 1168.8
49Ti
5"Ti
840.9
838.1
1
M-H, bend
; 2
502.0; 491.2; 486.9 500.3
3
;
'Isotopically mixed species. Photolysis of the matrix with light from 400 to 300 nm resulted in cleavage of the Sc-H and 0-H bonds. This caused a decrease in intensity of the a product bands and a parallel growth of the "b" peak at 954.6 cm-'. This band underwent only an oxygen-18 isotopic shift and is assigned to the metal monoxide, ScO. Its frequency is in good agreement with the previously reported gas-phase value12of 965 cm-' and an argon matrix value of 958 cm-'.13 The b band is shown in Figure lD, and its frequencies are listed in Table 111. In the D 2 0 experiments, photolysis of the a product caused a reduction in intensity of the Sc-D and ScOD stretching bands at 1070.0 and 698.2 cm-', respectively. In addition to generation of the ScO band at 954.8 cm-I, a strong band grew in at 699.2 cm-l which was not accompanied by a higher frequency band. This suggests that not all of the DScOD species was converted to ScO, but, instead, it formed an ScOD intermediate species. This band is labeled "d" in Figure 2B and listed in Table V. When the water:argon concentration was increased from a 6:lOOO to 20:lOOO ratio, while maintaining a constant metal concentration, the "c" bands grew in (Figure 1C). This behavior indicates the "c" product results from reaction between one metal atom and more than one water molecule. The large deuterium shift observed for the 1602.0-cm-l band indicates this is a metal hydrogen stretching mode. The position of this frequency, relative to the Sc-H stretching frequency in HScOH, suggests the metal (1 2 ) Huber, K.P.;Herzberg, G. "Constants of Diatomic Molecules"; Van Nostrand Reinhold: New Yorlc, 1979. (13) Weltner, Jr., W.; M c M , Jr., D.; Kasai, P.H.J. Chem. P h p . 1967, 46, 3172.
$
698 Sc + Water Figure 3. Expanded infrared spectra of Sc + water in an argon matrix immediately after deposition: (A) H2160,(B) H2I80/H2I60, and (C)
D20. is in a higher valence state, Le., with higher central charge on the metal, the metal-hydrogen stretching frequency is expected to increase. The 730.1-cm-' c band is close to the a product Sc-OH stretching frequency and was split into a triplet of bands in mixed H2I80/H2I60experiments (Figure 3). This splitting pattern is characteristic of a vibrational motion involving two equivalent OH groups. The isotopic shifts suggest assignment to a metal dihydroxy 0-M-O stretching frequency in a HO-Sc-OH group. The characteristics of the c product bands are consistent with assignment to the H$C(OH)~species. The c frequencies are listed in Table IV. The data are insulfficient for direct assignment of the number of hydrogen atoms. However, it is quite likely that only one hydrogen is present since the highest known oxidation state for scandium is 3+. This product was unaffected when irradiated with U V light. Titanium. Titanium metal atoms reacted spontaneously with water during cocondensation and generated the a bands shown in Figure 4A. The 1538.9-cm-' band exhibits a large deuterium shift, and is assigned to a Ti-H stretching mode. Photolysis of
Kauffman et al.
3550 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 I
I
I
I
I
A
v ''\
H
B
L
C I
1
'1' h
after
Ti
+
hv>400nm
I
Water
DtO
C
I I I ' 1600 1350 1 loo 850 600
c m-' Figure 4. Infrared spectra of Ti + H2160 in an argon matrix (A) immediately after deposition and (B) photolysis of matrix A with X 2 400
nm light.
1
the matrix with light in the region from 500 to 400 nm caused the a bands to disappear completely along with the appearance of the b product band (Figure 4B). Under high resolution the single b band is shown to be five peaks with frequencies and relative intensities indicative of a single titanium atom with a natural isotopic distribution. The metal and oxygen- 18 isotopic shifts and the absence of a deuterium shift indicate the presence of only one oxygen atom. The most intense peak has a frequency at 1010 cm-' which is in good agreement with the previous assignments to T i 0 of 1005 cm-' in a neon matrixI4 and 1000 cm-' in the gas phase.'* The a and b product frequencies are listed in Tables I1 and I11 and are assigned to HTiOH and TiO, respectively. The presence of the b band in the spectrum immediately after cocondensation, Figure 4A, indicates some of the HTiOH species had already decomposed to form TiO. The h products were present in the matrix spectrum immediately after depostion and were enhanced by photolysis (Figure 4B). The high-frequency h bands underwent only a large deuterium shift which is characteristic of a metal-hydrogen motion. The group of bands centered around 490 cm-' was not observed in the D20 spectra and apparently was shifted below the 400-cm-' lower limit of the spectrometer. Since the h bands were favored at low metal concentrations, it is reasonable to assume that they involve only one metal atom. Also, since they displayed no oxygen isotopic dependence, they are not caused by a water reaction. Previous experience with silicon atom r e a c t i o d 0 has shown that it is difficult to remove molecular hydrogen as a reactant from the system, and some hydrogen is also released from the formation of TiO. Thus assignment of these bands to various TiH, species is suggested. The h frequencies are listed in Table V. The c and g bands in Figure 4 were enhanced relative to the a product bands a t high water concentrations. The 843.5-cm-' g band exhibits the characteristic titanium isotopic distribution for a single metal atom. They also exhibited additional structure with oxygen-18 isotopic substitution when a small amount of oxygen-16 was present and a quintet in mixed H2I60D 2 0 experiments as seen in Figure SB, D. The mixed H2I60D 2 0isotopic behavior indicates there are three or more OH groups present. (14) Weitner, Jr., W.; McLeod,
Ti +Water
D.J . Phys. Chem. 1965, 69,3488.
I
8%
I
I
1
810
I
770
I
I
730
cm-1 Figure 5. Expanded infrared spectra of Ti + water in an argon matrix: (A) H2I60, (B) H2180, (C) D20,and (D) H2I60/D20.
No higher frequency bands appear to correlate with this band; thus, the absence of a M-H bond is suggested. The g bands are given in Table V and assigned to a Ti(OH), species where x is most likely 3 or 4. The c bands have a metal concentration dependence similar to the g bands and are also believed to contain only one metal atom. Each of the 806.0- and 77594x11-' bands splits into a triplet with oxygen-18 substitution. The deuterium splitting appears to be more complex when H20and D20 are mixed in approximately equal amounts, apparently due to the greater concentrations of the isotopically mixed species (Figure 5 ) . The 1670-cm-' c bands underwent a 480-cm-I deuterium shift and can be assigned to stretching modes of a TiH, group. The two stretching modes at 806 and 776 cm-' can be reasonably assigned to symmetric and antisymmetric stretching modes of a bent HOTiOH group. Thus a likely species assignment is HxTi(OH)2. The c band frequencies are listed in Table IV. Again, data are insufficient for assignment of the number of hydrogens, but the presence of two hydrogens with titanium in a tetravakent state seems the most likely. Vanadium. Vanadium metal atoms reacted with water immediately upon cocondensation to form the a product (Figure 6A). The 1583.0-cm-' a band exhibits a large deuterium shift and has been assigned to a V-H stretching mode. The 703.3-cm-' band exhibited oxygen-18 and deuterium shifts indicative of a M-OH stretching mode; see Figure 7A-C. The a bands are readily photolyzed away with X L 400 nm light as shown in Figure 6B. New bands appear at 987.2 cm-' for H2160and D 2 0 and 945.0 cm-' for H2I80near the b bands. In addition, growth of the b band is observed in most instances; however, this is not evident in Figure 6B. As was the case for the other metals the a bands have been assigned to the HVOH molecule and listed in Table 11. The appearance of the b band at 1029.7 cm-"on cocondensation, Figure 6A, is analogous to the appearance of the metal oxide band for scandium and titanium. This band exhibited no deuterium
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3551
Reactions of Sc, Ti, and V
TABLE VI
sc H-MOH k, mdyn/A Do: kcal DO,k j
1526.9 20.9 1.34 77 322
k, mdyn/A
3.73
we WCXC
Ti
V
1580.5 20.8 1.44 84 351
1633.1 25.1 1.54 74 310
3.79
3.84
DM-OD a Linear Birge-Sponer extrapolation from pseudodiatomic constants; see Discussion.
dicates that some of the hot H M O H species further dissociate to MO H, or 2H. Alternatively, light from the hot furnace walls may be capable of photolyzing the H M O H species. This is suggested by our photolysis experiments which demonstrated that relatively low-energy light, Le., in the region 540-400 nm, will cleave the hydrogen bonds in the H M O H molecule. The photolysis reaction of the DScOD molecule differed from the other HMOH and DMOD molecules by forming ScOD as well as ScO. This would explain the usual behavior seen by Liu' in the gas-phase oxygen abstraction process of Sc and Y with D20. Liu reported that the formation of M O in the deuterium experiment was much less than expected, relative to the yield when H 2 0 was used. Preferential formation of ScOD would be in agreement with this discrepancy. A simple diatomic model has been applied to the calculation of isotopic shifts for the M-O stretch in the H M O H and M O species. Both the H M and OH groups are treated as a single mass. This is expected to be a reasonable approximation and allows one to check for unusual shifts. One notices a significant lack of agreement for the oxygen- 18 shifts; Le., the calculated shifts are larger than those observed. For deuterium substitution the observed shift deviates very significantly from the calculated value for HTiOH and HVOH. The isotopic behavior of this mode suggests that it is mixing with other modes or combinations of modes. The modes which are available are the two low-frequency hydrogen bending modes. However, for these modes to possess the same symmetry as the HM-OH stretching mode, the H M O H molecule must be nonlinear. Thus the observed isotopic shifts offer proof of a nonlinear H M O H geometry. An analysis of the frequency data for the HMOH species which utilizes the diatomic approximation for both the H-MOH and HM-OH bonds has been made. This is thought to be valid for illustration of force constant trends and even bond energy trends for the H-MOH bond. Calculation of a pseudodiatomic we and uexefor the H-M bond assumes that we varies with p-'I2 and uexe as p-l where I.( is the pseudodiatomic reduced mass. Calculation of H-MOH bond energies with the linear Birge-Sponer approximation,' Do = (w,2/4w,xF) - '/,we, provides a means of representing the expected variation in H-MOH bond energies as indicated by the calculated anharmonic constants. The calculated bond energies are most likely higher than the true bond energy values., Table VI gives the calculated values. It is seen that both the H-MOH and M-OH force constants increase with atomic mass while the calculated bond energy of the H-MOH bond shows a slight peaking at titanium. The calculated variation in bond energy can be explained by the filling of increasingly destabilized d-6, d-r, and d-a orbitals. The increase in the hydrogen-bond force constant with the atomic number of the metal can be attributed to the decreasing size of the bonding metal orbitals with atomic number across the transition-metal series. This should result in a decreasing metal-hydrogen bond length with atomic number. Thus, even if the bond energy remains constant, the force constant is expected to increase. The calculated oxygen-18 shifts for the M O species are in reasonable agreement with the measured values as seen in Table 11. It is interesting that the measured frequencies for Ti0 and
+
1525
1275
1025
775
525
cm-l
Figure 6. Infrared spectrum of V + H2160in an argon matrix (A) immediately after deposition and (B) after photolysis, X > 400 nm.
i
I
-788
670
a
-k-J647
C
a V +WATER i W N MATRIX)
Figure 7. Expanded infrared spectra of V + water in an argon matrix (A, D) H2160, (B, E) H2180 H2I60, and ( C , F) H2I60D2,0.
dependence, and the oxygen-18 isotopic shift indicated the presence of one oxygen atom. It is assigned to the VO molecule, in agreement with the gas-phase value of 1002 cm-I.l2 In addition, the 987.2-cm-l band which also grew in with photolysis is thought to be due to a slightly perturbed VO molecule where the perturbing species is a water molecule or carbon monoxide. The carbon monoxide results from reaction of the metal oxide coating on the sample with the carbon container during vaporization of the metal. The b frequencies are listed in Table 11. The c band, Figure 6, at 787.6 cm-' was split into a set of triplet bands in both the H2I80and D,O experiments as shown in Figure 7D-F. This splitting pattern is that expected for two equivalent OH groups of a H O - V a H group. The 1601.9-cm-I band shows a large deuterium shift and is assigned to a H,V stretching mode. This behavior suggests the c bands can be assigned to the H,V(OH), species where x is 1 or 2. The c frequencies are listed in Table IV.
Discussion Scandium, titanium, and vanadium metal atoms reacted spontaneously with water on cocondensation at 15 K to form H M O H molecules. In all three cases, there was no evidence of a metal atom-water molecule adduct as previously found for some other metals.34 The most reasonable explanation is that the adduct is short-lived and rapidly rearranges to form the H M O H molecule. The formation of M O species without photolysis in-
J. Phys. Chem. 1985,89, 3552-3555
3552
VO in an argon matrix are slightly above their gas-phase values. For T i 0 the value of 1005 cm-’ in a neon matrix is also above the gas-phase v a 1 ~ e s . lThe ~ opposite shift is observed for most diatomic molecules as might be expected due to partial solvation of the molecule by the matrix. A shift to higher frequencies suggests that some internal bonding changes other than simple solvation are induced by the matrix. Formation of the H$e(OH)2, H,Ti(OH)z, and H,V(OH)2 species must result from the reaction of one metal atom with two water molecules. This reaction was complete upon cocondensation since no additional product was formed as a result of photolysis; however, there is no evidence to distinguish between a sequential or concerted reaction. Formation of Ti(OH), where x 2 3 is surprising since it indicates facile reaction with three or more water molecules. Reaction with a trimer or tetramer of water seems most likely, but a series of reaction steps is possible. The following equations summarize the reactions observed for atomic Sc, Ti, and V with water in solid argon.
IS K
M + H @ , r
M t 2H20
15 K 7
Ti t n t i g
CM*OH21*
CM*(OH2)23*
IS K 7 HMOH
-
*,
MO t Hz
H,M(OH)2
-
CTi*(OHz),?
Ti(OH),,
n_>3
The asterisk denotes “not observed”,
Acknowledgment. The authors are pleased to acknowledge the financial support of this work by the National Science Foundation and by the Robert A. Welch Foundation. Registry No. H20, 7732-18-5; ScO, 12059-91-5;ScOD, 96759-94-3; TiO, 12137-20-1; HTiOH, 81514-91-2; VO, 12035-98-2; V(OH),, 39096-97-4; HSCOH,75594-09-1;HVOH, 81514-92-3;H2180, 1431442-2; DzO, 7789-20-0; Ti(OH),, 12026-77-6; Ti(OH),, 20338-08-3; TiHx, 11 140-68-4;Sc, 7440-20-2; Ti, 7440-32-6; V, 7440-62-2.
Determination of Proton-Transfer Rates and Energetics for the Clathrate Hydrate of Oxirane by FT- I R Spectroscopy Hugh H. Richardson, Paul J. Wooldridge, and J. Paul Devlin* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: February 20, 1985)
The FT-IR spectroscopicobservation of proton-transfer rates has been extended to the structure I clathrate hydrate of oxirane. Methods have been developed for isolating DzO intact in the crystalline clathrate at 100 K. Kinetic data for the 105-120 K range indicate direct conversion of DzO to isolated HOD, a result that identifies the orientational defect as the majority charge carrier. The small activation energy for this exchange reaction (-5 kcal) has been tentatively related to the difference between the energy required to mobilize protons trapped by the abundant L-defects (- 12.7 kcal) and the activation energy for formation of the L-defects (-7.7 kcal). The surprising abundance of mobile protons at 110 K has been rationalized by invoking ion-defect formation and trapping during the epitaxial growth of the clathrate samples. Radiolysis effects (1.7-MeV electrons) have added to the understanding of defect behavior in the oxirane clathrate and have permitted the observation of a distinct spectrum for neighbor-coupled (HOD)z units.
-
Introduction The spectroscopic determination of proton-transfer rates for cubic ice has recently provided molecular-level insights to a process that had previously been extensively investigated only by more classical bulk techniques such as conductivity and dielectric constant measurements.’ These new results have been possible because of the development of methods for preparing crystalline cubic ice, at 125 K,containing isolated intact D 2 0 molecules as part of the H-bonded ice network. A kinetic study of the formation of neighbor coupled HOD units, (HOD)2, and isolated HOD from the isolated DzOfor the temperature range from 135 to 150 K yielded quantitative values for the activation energy of proton hopping (9.5 kcal) and orientation defect migration (12.0 kcal) as well as several instructive qualitative insights. Among the latter perhaps the most interesting were firm evidence that (a) the mobility of the hydroxide ion in cubic ice is several orders of magnitude less than that of the proton and (b) the ion pair defects and the Bjerrum orientational defects are comparably effective charge carriers for the temperature range near 150 K. Similar methods have more recently been developed for the isolation of DzO molecules in the H-bonded network of certain crystalline clathrate hydrates so that a similar kinetic study of these substances is possible.2 It is well-known that the host
-
(1) W. B. Collier, G. Ritzhaupt, and J. P. Devlin, J. Phys. Chem. 88, 363 (1984).
-
water-network structure for the clathrates is icelike in many respects with the average H-bond only 1 .O% longer (weaker) than for ice.3 However, the orientational relaxation rates of the host lattice, which varies somewhat depending on the clathrate guest species, tends to be orders of magnitude greater than for cubic ice.4 Based on the somewhat weaker H bonds and the greater orientational relaxation rates, it is reasonable to anticipate significant differences in results for the thermally induced conversion of the isolated D 2 0 molecules to (HOD)2 and HODimlatd, for the clathrates, compared to the published results for ice. Proton transfer through an H-bonded system apparently occurs by the alternate ”hopping” of a proton ion defect and the passage of an orientational (Bjerrum) L-defect. The hopping step is believed to resemble a small-polaron motion5,6and occurs with a relatively small activation energy that may be positive or negative depending on the temperature in question. Thus the hopping rate, and the associated activation energy, reflects primarily the (2) J. E. Bertie and J. P. Devlin, J. Chem. Phys., 78, 6340 (1983). (3) D. W. Davidson, “Water, A Comprehensive Treatise”, Vol. 2, F. Franks, Ed., Plenum Press, New York, 1973, Chapter 2. (4) D. W. Davidson, S.K. Garg, S. R. Gough, R. E. Hawkins, and J. A. Ripmeester, “Inclusion Compounds”, Vol. 11, Eric Davies, Ed., Academic Press, in press. ( 5 ) M. Kunst and J. M. Warman, J. Phys. Chem., 87, 4093 (1983). (6) K. Kawabata, Y. Nagata, S. Okabe, N. Kimura, K. Tsumari, M. Kawanishi, G. V. Buxton, and G. A. Salmon, J . Chem. Phys., 77,3884 (1982).
0022-3654/85/2089-3552$01.50/00 1985 American Chemical Society
