J . Phys. Chem. 1992, 96, 5746-5748
5746
Photochemical and Thermochemical Reactions of Mn04- Ions in NH4CI04and NH4BF4 Crystals: An EPR Study Jiang-Tsu Yu Institute of Physics, National Taiwan Normal University, Taipei 1 1 718, Taiwan, Republic of China (Received: September 23, 1991; In Final Form: March 25, 1992)
UV photochemical and thermochemical reactions of the Mn0; ions doped into single crystals of NH4C104and NH4BF4 have been investigated via the method of electron paramagnetic resonance (EPR). An isotropic EPR spectrum displaying a 55Mnhyperfine sextet has been observed in both reactions. The EPR spectra of the Mn(I1) species produced by UV irradiation and by thermal treatments are identical and identical to that exhibited by the doped Mn2+ions. The g-factor is g = 2.004 and the hyperfine constant is A" = 96.6 G in NH4C104for the Mn2+ions produced by thermal treatments, by W irradiation, and by doping. The corresponding values are g = 2.008 and Ass = 94.2 G for NH4BF4. The observed MnV"O; to Mn(I1) reduction can be regarded as the reduction part of an overall redox reaction between Mn04- and the NH4+group. We discuss the mechanism of the solid-state redox reaction.
1. Introduction The adsorption spectrum of Mn04- consists of three bands centered about 550 and 340, and below 200 nm.1-3 These bands have been explained and analyzed by Wolfsberg and Helmholz4 in terms of a molecular orbital calculation. The primary UV photochemical reaction of aqueous MnO; is a simple dissociation into two fragments' Mn04-
+ hv
-
Mn02
+ 02(g)
(1)
where the position of the extra electron was left uncertain. Zimmermanl explained the mechanism of dissociation as such that the photoexcited Mn04- undergoes a radiationless transition to a high vibrational level of the ground state, and this internal conversion produces "hot" Mn04- ions which then dissociate thermally according to eq 1. Klaning and Symons5 have studied by electron paramagnetic resonance (EPR) the photolysis of Mn04- in aqueous solutions and in frozen alcohol glasses. They concluded that the photolysis proceeds to the stage of Mn3+(MnOz-), but no further. This paper reports the UV photochemical and thermochemical reactions of the Mn04- ions doped into single crystals of the isomorphous NH4C104and NH4BF4. We have detected by EPR Mn(VI1) to Mn(I1) reductions. This reduction can be regarded as the reduction part of an overall redox reaction between Mn04and the NH4+group. We will present our experimental results and discuss the mechanisms of the chemical reactions. NH4C104 (AP) crystallizes in an orthorhombic Pnma structure.6 There are four molecular units within a unit cell, and the NH4+ groups and the C104- ions are all chemically equivalent. Because of its use in solid rocket propellants, the physicochemical properties of AP have been extensively investigated. Jacobs and Whitehead7 have reviewed the decomposition and combustion of AP. AP is stable at room temperature but decomposes at measurable rates at T I 150 0C.7 AP undergoes an orthorhombic to cubic transition at about 240 OC. The 240 O C transition is endothermic and is accompanied by a significant amount of decomposition. The Mn(VI1) to Mn(I1) thermal reduction that we have observed in NH4C104:Mn04-crystals can be carried out via thermal treatment a t temperatures well below the 240 "C phase transition. NH4BF4crystallizes in a structure similar to that of AP, but not much is known about its physicochemical properties. 2. Experimental Section Single crystals of NH4C104and NH4BF4doped with Mn04were grown from aqueous solutions held at 20 "C. Because KMn04 is isomorphous with these two compounds, Mn04- can be readily doped. NH4C104:Mn04-crystals are purple in color, and NH4BF4Mn04-crystals are light purple or pink in color. TO make sure than no Mn2+ impurity was accidentally doped into
the sample crystals, we made EPR measurements prior to UV irradiation or thermal treatments. The experimental results were negative, so we can state that the MnZ+ions we have detected by EPR were produced either via UV irradiation or thermal treatments. For comparison, we have also grown NH4C104 and NH4BF4crystals doped with Mn2+ and have investigated their EPR spectra. We have also grown KC104:Mn04- crystals from aqueous solutions held a t 20 O C . Because KC104 is also isomorphous with NH4C104 and KMn04, Mn04- can be readily doped into KC104 crystals. KC104:Mn04- crystals are purple in color. UV irradiation was carried out a t room temperature using a Bruker 200.W UV irradiation system. The crystals were irradiated in front of the UV lamp at room temperature. A chromel-alumel thermocouple was used to measure the temperature of the crystal during the course of the UV irradiation. We found that the temperature increased by only a few degrees, even for long periods of irradiation. We have also experimented by placing an IR filter (a water column sandwiched between quartz plates), a band-pass filter (220-420 nm), or a high-pass filter (1260 nm) in front of the W lamp. The experimental results with filtered and unfiltered UV irradiation were the same. Thermal treatments were carried out inside a programmable furnace. We used a Bruker ER 200 D, X-band spectrometer to make EPR measurements. 3. Results and Discussion
Untreated NH4C104.Mn04- crystals did not exhibit an EPR spectrum. This also implies a UV photochemical reduction of Mn7+ (in Mn04-) into Mn2+. After UV (filtered or unfiltered) irradiation at room temperature for 25-50 h, an EPR spectrum displaying the six hyperfine lines of the isotope 55Mn can be detected at room temperature (seeFigure 1). On the other hand, KC104:Mn04- crystals UV-irradiated for up to 68 h had not yielded MnZ+ions detectable by EPR. This spectrum is isotropic. This indicates that the UV-induced Mn(VI1) to Mn(I1) reduction is accomplished with the assistance of the N&+ group. NH4C104 crystals doped with Mn2+ ions exhibited an identical EPR spectrum (see Figure 1). For ease of discussion, we refer to the Mn2+ ion produced by UV irradiation as the photolyzed Mn2+and that produced by thermal treatment as the pyrolyzed Mn2+, so as to distinguish them from the doped Mn2+. This isotropic spectrum can be regarded as the 55Mnhyperfine lines of the +'I2 transition of the S = manifold of Mn2+ (3d5). The yield of the photolyzed Mn2+is stable and accumulative. The g-factor is g = 2.004, and the hyperfine constant is A 55 = 96.6 G. The W efficiency is apparently low, because NH4C104:Mn04-crystals remained purple in color after 50 h of irradiation. The experimental results shows that the UV produced paramagnetic species related to Mn is Mn2+. The yield of the photolyzed MnZ+is very
0022-365419212096-5746%03.00/0 0 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5141
Mn04- Ions in NH4C104 and NH4BF4
3200G 3000G
3800G
Figure 1. (a) EPR spectrum of UV-photolyzed (UV-irradiated at room temperature for 50 h) Mn2+ions in an NH4CI04:Mn0; crystal observed at 304 K. (b) Room-temperature EPR spectrum of doped Mn2+ions in NH4CI04crystals. The microwave frequency was about 9.49 GHz. The sharp peak near g = 2 is due to an unidentified defect species in the quartz dewar insert (same for Figure 2).
small (for 50 h of irradiation) for the cases when a band-pass filter (220-420 nm) or a long-pass filter (1260 nm) was used. This indicates that UV lights with wavelengths smaller than about 220 nm are most effective for the production of photolyzed Mn2+. This result is consistent with the observation by Zimmerman' that the quantum yield of the Mn04- Mn02- O,(g) photolysis decreases monotonically with increasing wavelength. NH4C104:Mn04- crystals after having been thermally treated a t 150-190 OC for 2-4 h exhibited an EPR spectrum identical to that of doped Mn2+ or photolyzed Mn2+. For thermal treatments at temperatures below 150 OC, the results were sample dependent. For example, one of the crystals thermally treated a t 80 OC for 20 h did not yield pyrolyzed Mn2+ ions detectable by EPR, whereas another crystals thermally treated at 80 OC for 23 h exhibited a weak EPR spectrum of pyrolyzed Mn2+. As a rule, for thermal treatments at temperatures below about 150 OC, the yield of the pyrolyzed Mn2+ions is small a t best. This result can be correlated with the observation that the thermal decomposition of NH4C104becomes significant above 150 OC.' It is interesting to note that for the same duration of treatment, the yield of the photolyzed Mn2+ (as judged by the EPR signal amplitude) is much greater than the yield of the pyrolyzed Mn2+ions for thermal treatment at 80 OC. This shows that the U V - i n d u d Mn(VI1) to Mn(I1) reduction is defmitely photochemical in origin. Furthermore, KC1O4:Mn04 crystals thermally treated first at 180 "C for 4 h and then at 200 O C for 2 h did not exhibit an EPR spectrum of pyrolyzed Mn2+ ions. This demonstrates that the pyrolyzed Mn2+ ions were produced with the assistance of the NH4+groups. The photochemical and thermochemical dissociations of the Mn0; ions in NH4BF4crystals are similar to those in NH4C104. NH4BF4.Mn04-crystals irradiated with UV (unfiltered or IRfiltered) for 40-50 h exhibited (see Figure 2) an isotropic EPR spectrum similar to that of the Mn2+ions in NH4C1O4. The EPR spectrum of the photolyzed Mn2+ions in NH4BF4is also identical to that exhibited by doped Mn2+ or pyrolyzed Mn2+. A fairly strong EPR spectrum of photolyzed Mn2+(for 20 h of irradiation) was detected for the case when a band-pass filter (220-420 pm) was used, though the yield is smaller than that for unfiltered irradiation for the same duration. When a long-pass filter (2260 nm) was used, the yield of photolyzed Mn2+for 50 h of irradiation is about of that for 20 h of irradiation with the use of the band-pass filter. These results again demonstrate that shortwavelength UV lights are most effective in the production of photolyzed Mn2+. The g-factor is g = 2.008, and the hyperfine constant is 94.2 G . The efficiency of the UV production of photolyzed Mn2+ in NH4BF4 is also apparently low, since the crystal remained purple in color after 50 h of irradiation. The UV-induced Mn(VI1) to Mn(I1) reduction in NH4BF4 is also photochemical in origin, because crystals thermally treated at 70 OC for 50 h did not exhibit an EPR spectrum of pyrolyzed Mn2+. However, crystals thermally treated at 70 O C for 6 days yielded
-
+
3400G
3600G
Figure 2. (a) EPR spectrum of UV-photolyzed (for 50 h of UV-irradiation at room temperature) Mn2+ions in an NH4BF4:Mn0; crystal. (b) EPR spectrum of pyrolyzed Mn2+ions (thermally treated at 180 OC for 4 h) in an NH4BF4:Mn0ccrystal. (c) EPR spectrum of doped Mn2+ ions in an NH4BF4crystal. These three EPR spectra were taken at room temperature and at a microwave frequency of about 9.49 GHz.
pyrolyzed Mn2+ions detectable by EPR, but the spectrum amplitude is about 1 order of magnitude smaller than that exhibited by the photolyzed Mn2+ ions for 50 h of UV irradiation. The site symmetry and ligand arrangement of the Mn2+ ions in NH4C104and NH4BF4cannot be inferred from the observed EPR spectra. However, considering the crystal structures, the site symmetry should be rhombic or lower. The peak-to-peak line width is large (23 G) at 300 K for the Mn2+ ions in both compounds. Because the ionic radius of MnZ+is much smaller than that of NH4+, doped Mn2+ in NH4C104 and NH4BF4 is more likely to be interstitial than substitutional. In the crystal lattice of NH4C104,each anion is surrounded by seven cations and vice versa.* The coordination in this structure has been described as indefinite.6 Since the pyrolyzed and the photolyzed Mn2+ions are reaction products of redox reactions, it is very probable that these ions are associated with local H 2 0molecules produced by the oxidation of the NH4+groups. Because the EPR spectrum of the doped Mn2+ion is identical to those of the pyrolyzed and the photolyzed Mn2+ions, the structure of these three kinds of ions should be the same. It can be assumed that at the completion of the UV or thermal reduction, the produced Mn2+ is forced to move away from the anionic site occupied by Mn7+ of Mn04- to an interstitial site. The large peak-to-peak line width (23 G) observed for the Mn2+ ions in NH4C104and NH4BF4probably prevented the fine-structure lines from being detected. The mechanism of the photoredox reaction can be considered to consist of two stages: a UV-photon induced dissociation of the type MnOC + hu MnOy 02(g), followed by a redox reaction between M n 0 y and NH4+. It is also possible that the redox reaction could proceed without the self-dissociation of Mn04- as a precursor. A portion of thc UV photon energy absorbed by Mn04- can be transferred to its local NH4+ groups via coupled lattice vibrations. The excited NH4+ groups then react with either Mn02- or Mn04- to produce MnZ+. Furthermore, no UV-photolyzed or pyrolyzed Mn2+ can be detected by EPR in KC104 crystals cocrystallized with NH4C104and KMn04. This tends to suggest that the NH4+ groups participating in the redox reactions in NH4C104and NH4BF4are local to Mn04-. The mechanism of the thermochemical reaction can be regarded as similar to that of the photochemical reaction. Mn0; dissociates first into Mn02- and oxygen gas. This is followed by a redox reaction between Mn02- and NH4+. Electrical conductivity due to free protons released by the thermally excited NH4+ groups has been observed for example in (NH4)2S04and LiNH4S04 crystal^.^,^^ The thermal dissociation of Mn04- into MnZ+in (NH4)2S04crystals has been investigated recently via EPR by Yu and Chou." Recently, we have observed an EPR spectrum of Mn2+ ions in (NH4)2S04-Mn04-crystals stored in the dark in desiccators at ambient temperatures for over 1'/2 years. This EPR spectrum, which is identical to that of the thermally produced Mn2+ions and to the doped Mn2+ions, is otherwise not detectable in freshly-grown crystals. This phenomenon can be attributed to the high oxidation power of Mn04- and to the chemical in-
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+
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J . Phys. Chem. 1992, 96, 5748-5752
stability of the ammonium group in certain ammonium compounds. It can be postulated that even at room temperature, free protons and free electrons are being released by the ammonium group in (NH4)2S04crystals, which then react with Mn04- to produce Mn2+. The rate of this Mn2+ production at room temperature is slow but accumulative such that it can be eventually detected by EPR. It has been shownI0 that free protons released by the ammonium group contributes to the electrical conductivity of LiNH4S04crystals in all of its known phases, including the room-temperature and the low-temperature phases. We suggest that these free protons and free electrons participated in the room-temperature reduction of Mn04- into Mn2+ in (NH4)2S04:Mn0, crystals. However, crystals of NH4C104:Mn04-and NH4BF4:Mn04-stored in the dark for more than 1 year did not exhibit any EPR spectrum of Mn2+. This tends to suggest that the ammonium groups in these two ammonium compounds are chemically more stable than those in (NH4)2S04.
Science Council (NSC) of the Republic of China under Project NO. NSC8 1-0208-M003-06. Registry No. Mn04-, 14333-13-2; NH4C104,7790-98-9; NH4BF4, 13826-83-0; Mn2+, 16397-91-4.
References and Notes (1) Zimmerman, G. J . Chem. Phys. 1955, 23, 825. (2) Ada", A. W.; Waltz, W. L.;Zinato, E.; Watts, D. W.; Fleischauer, P. D.; Lindholm, R.D. Chem. Reu. 1968,68, 541. (3) Carrington, A.; Symons, M. C. R. J . Chem. SOC.1956, 3373. (4) Wolfsberg, M.;Helmholz, L. J . Chem. Phys. 1952, 20, 837. (5) Klaning, U.;Symons, M. C. R. J . Chem. SOC.1953, 3580. (6) Wyckoff, R. W. G. Crysral Srrucrures, 2nd ed.; Interscience: New York, 1965; Vol. 3. (7) Jacobs, P. W. M.; Whitehead, H. M. Chem. Reu. 1969, 69, 551. (8) Chakraborty, T.; Khatri, S.S.; Verma, A. L. J . Chem. Phys. 1986,84, 7018. (9) Syamaprasad, U.;Vallbhan, C. P. G.Solid Srate Commun. 1981,38, 555.
(10) Syamaprasad, U.;Vallabhan, C. P. G. Phys. Rev. 1982,826,5941. (11) Yu, J. T.; Chou, S.Y. J. Phys. Chem. Solids 1990, 51, 1255.
Acknowledgment. This research is supported by the National
Ab Initio Study of the Identity of the Reaction Product between C3 and Water in Cryogenic Matrices Ruifeng Lib* Xuefeng Zhou, and Peter Pulay* Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701 (Received: October 8, 1991)
Geometry optimization, energy calculation, and ab initio vibrational analysis were carried out on different conformers of singlet and triplet hydroxyethynylcarbene and 3-hydroxypropadienylidene. They are probable products of the reaction C3 + H20. Theoretical methods used include restricted Hartree-Fock, unrestricted Hartree-Fock natural orbital-complete active space, and Moller-Plesset perturbation theory. By comparing the calculated harmonic frequencies (empirically scaled by 0.9) with observed FTIR frequencies of the product of the reaction C3 + H20 in cryogenic matrices, it appears likely that the product is singlet 3-hydroxypropadienylidene instead of the previously proposed hydroxyethynylcarbene.
I. Introduction In the past several years, there has been increased interest in the study of physical and chemical properties of all carbon clusters.'+ While most of the experimental efforts are focused on synthesis and characterization of larger and larger clusters, little is known about their chemical reactivity. Theoretical calculations on the structures, energetics, and reactivities of small carbon clusters have been shown to be very helpful in elucidating the physical and chemical For example, C, was thought to be linear for a long time, but ab initio calculations concluded that it is bent with an equilibrium angle 162' and a small barrier of about 21 an-'to linear it^.^,^ Recent experimental w0rk8,~confirmed the nonlinear character of the C3 structure, and the angle and the barrier to linearity were deduced to be 162.5O and 16.5 cm-I, in excellent agreement with ab initio predictions. Recently, elegant experimental work was conducted on the reactivity of small carbon clusters ranging from CI to C5toward water in cryogenic matrices.1° On the basis of the matrix FTIR spectra, it was concluded that neither ground-state Cl nor C2forms stable adducts or products under the experimental condition. This is in agreement with ab initio prediction and gas-phase experimental conclusion that it is C('D) and not C(3P) which reacts with H 2 0 to form C O + H2 and formaldehyde.' Under the same condition in an argon matrix, C3 forms an addrlct with water without activation. This C3(H20) complex undergoes a sequence of photochemical reactions. Upon irradiation with 400-nm light,
a unique intermediate is formed and isolated. This intermediate photorearranges with 280-nm ultraviolet light into propynal, which was previously identified by the gas-phase infrared spectrum. I Based on the observed FTIR frequencies and l80isotope shifts, the intermediate was proposed to be hydroxyethynylcarbene (HEC).'O Carbene is one of the most important transient molecules. It has been well studied both theoretically and experimentally.I2 But hydroxyethynylcarbene had not been reported before. To characterize the structure, energetics, and harmonic vibrations, we carried out ab initio calculations and vibrational analysis on stable conformers of both singlet and triplet states. To our surprise, the calculated frequencies are not in reasonable agreement with the recorded matrix FTIR frequencies. To determine the identity of the product of C3 with water, stable conformers of the singlet and triplet 3-hydroxypropadienylidene were also studied by the ab initio methods. The latter molecule has not been reported either, but vinylidenecarbene, an isomer of C3H2 and structurally similar to 3-hydroxypropadienylidene,was generated by photolysis of cyclopropenylidene in a matrix and identified by comparing the observed infrared spectrum with results of ab initio calculati~ns.'~ Both hydroxyethynylcarbene and 3-h ydroxypropadienylidenc have the same stoichiometry, and both become propynal by a single hydrogen migration; therefore, both are probable reaction products. Our calculated results indicated that it is probably singlet 3hydroxypropadienylidene which was generated in the reaction of
0022-365419212096-5748$03.00/00 1992 American Chemical Society
