J . Phys. Chem. 1991, 95, 514-518
514
The mass difference between H and D atoms seriously affects the tunneling rate, and the amount can be approximately estimated by eq 2. When the vibration between N and H / D atoms is assumed to be a harmonic oscillator, the mass effect for the line width between H and D atoms is estimated by the factor T ( H / r D
rH/rD
(mD/mH)1/2exp[ (mD’/’ - mH’/’) x (“(V-
€LX)l’/’
dr]] ( 3 )
in consideration of both the vibrational frequency and the factor F ~ in/ eq~ 2. Here, mDand mH are the mass weight of D and H atoms, respectively. This factor corresponds to the mass effect
for the potential tunneling between H and D atoms. At the ~ 2 ) = 1 vibrational level of ND, the amount of the factor (3) is about 6, so that the exponent factor amounts to 4.2. This value should be connected with the potential energy function, V(r),and a similar analysis for the 0; and 2; transitions can reveal details of the dissociation mechanism on the potential energy surface. The analysis for the 08 and 2; transitions and an ab initio calculation are in progress in our group.
Acknowledgment. The authors are grateful to Professor S. Iwata for valuable discussion. The financial support from Grant-in-Aid for Scientific Research for Priority Area by the Ministry of Education is greatly acknowledged.
Femtosecond Infrared Spectroscopy: Ultrafast Photochemistry of Iron Carbonyls in Solution Philip A. Anfinrud: Chul-Hee Han, Tianquan Lian, and Robin M. Hochstrasser* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania I9104-6323 (Received: May 18, 1990)
The photochemistry of cyclopentadienyliron dicarbonyl dimer in cyclohexane has been investigated with a novel femtosecond infrared spectrometer. Visible (580 nm) photolysis proceeds through an excited bound state that persists for about 1 ps. Dissociation occurs through more than one channel and is suggested to generate singly bridged and/or nonbridged dimers and homolytically cleaved species. Solvation of the nascent fragments apparently begins around 5 ps and nears completion around IO ps. This delay in solvation may arise because visible excitation generates photoproducts with sufficient excess energy to inhibit complexation with the solvent at earlier times. Roughly half of the photodissociated dimers recombine geminately within =20 ps, presumably through a singly bridged intermediate. Therefore, singly bridged photoproducts would not participate in photoinitiated bimolecular chemical reactions with diluted species.
of organometallic compounds. For example, what excited states Introduction are involved, what is the quantum efficiency for photodissociation, We recently introduced a new method for obtaining IR spectra what are the nascent fragments, how might the solvent affect the with femtosccond time reso1ution.l This development has enabled partitioning among available reaction pathways, is there any an ultrafast investigation into the photochemistry of [CpFe(CO),], geminate recombination, and what is the mechanism and rate of (Cp = q5-C5H5)in cyclohexane where the primary photoproducts solvation? This problem was recently addressed in this laboratory and their dynamics have been observed for the first time. Although by using transient IR spectroscopy with -30-ps time resolution.I0 the photochemistry is complex, the time-resolved IR spectra are While these picosecond spectra provided evidence for new insufficiently detailed to illuminate some of the primary phototermediates, not even that time resolution was sufficient to chemical reaction pathways. characterize the primary processes. The extension of IR specCoordinatively unsaturated organometallic complexes are thought to be reactive intermediates in homogeneous c a t a l y s i ~ . ~ ? ~ troscopy into the femtosecond regime has made the current investigation possible. Such species are readily generated upon photolysis of transition-metal carbonyls. Hence, characterization of photochemical reaction pathways for saturated organometallic species is par(1) Anfinrud, P. A.; Han, C.; Hanscn, P. A.; Moore, J. N.; Hochstrasscr, ticularly relevant to the understanding of homogeneous catalysis. R. M. In Ultrafast Phenomena; Springer: Berlin, 1988; Vol. VI, p 442. The photochemistry of dinuclear transition-metal carbonyls has (2) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic Press: New York, 1978. been extensively studied by numerous IR techniques. Near-UV (3) Parshall, G. W. Homogeneous Catalysts: The Application and irradiation of [CpFe(CO)z]2in an organic matrix generated only Chemistry of Catalysis by Soluble Tramitlon Metal Complexes; Wiley: New one photoproduct, which was identified through its IR spectrum York. 1980. - ----, -- -as a CO-loss product with three bridging CO bond^.^.^ A (4) Hooker, R. H.; Mahmoud, K. A.; Rest, A. J. J . Chem. Soc., Chem. Commun. 1983, 1022. time-resolved IR investigation in cyclohexane reported a second ( 5 ) Hepp, A. F.; Blaha, J. P.; Lewis, C.; Wrighton, M. S.Organometallics intermediate, the homolytically cleaved dimer! It was found that 1. 17A -lWd - - ., -, - . .. visible excitation provides sufficient energy for homolytic cleavage, (6) Moore, B. D.; Simpson, M. B.; Poliakoff, M.; Turner, J. J. J . Chem. Soc., Chem. Commun. 1904, 912. but near-UV photolysis is required to form the triply bridged ( 7 ) Dixon, A. J.; Healy, M. A.; Hodges, P. M.; Moore, B. D.; Poliakoff, intermediate. While microsecond time-resolved IR spectroscopy M.; S i m p n , M. B.; Turner, J. J.; West, M. A. J. Chem. Soc.,Fmday Tram. has contributed greatly to our understanding of photoinitiated 2 1986,82,2083. diffusion-controlled chemistry,H this technique is unable to ad(8) Moore, B. D.; Poliakoff, M.; Turner, J. J. J . Am. Chem. Soc. 1986. dress issues related to the primary events in the photodissociation 108, 1819. ~
‘Present address: Department of Chemistry, Harvard University, 12 Oxford St., Cambridge, MA 02138.
~~~
(9) Dixon, A. J.; Hcaly, M. A,; Poliakoff, M.; Turner, J. J. J. Chem. Soc.. Chem. Commun. 1986, 994. (10) .Moore, J. N.; Hansen, P. A.; Hochstrasser, R. M. J . Am. Chem. Soc. 1989, 1 1 1 , 4563.
0022-3654191 /2095-0514302.50/0 0 1991 American Chemical Society
Photochemistry of Iron Carbonyls Experimental Section A femtosecond IR spectrometer1 has been used to monitor the photodissociation dynamics of cyclopentadienyliron dicarbonyl dimer, [CpFe(CO),],, at room temperature in cyclohexane. Briefly, a sample is pumped with a visible pulse and probed with continuous-wave (CW) IR light from a tunable diode or CO laser. Gated detection of the C W IR probe is achieved with femtosecond time resolution by frequency summing the transmitted IR with an ultrashort visible pulse in a nonlinear crystal. The upconverted signal is proportional to the transmitted IR intensity over the slice in time defined by the gating pulse. Time-resolved IR spectra are obtained by recording the differential IR absorbance (pumped vs unpumped) as a function of probe wavelength with a fixed optical delay between the pump and gating pulses. The time evolution of a particular feature is measured by scanning the optical delay. The time resolution of this technique is determined by the cross correlation of the pump and gating pulses. When these pulses are derived from the same laser, the time resolution is limited only by the optical pulse duration. The spectral resolution is determined by the spectral bandwidth of the probe. Hence, this novel approach permits two-color pumpprobe spectroscopic investigations with ultrashort time resolution without sacrificing spectral resolution. The optical pulses (580 nm) originate in a cavity-dumped rhodamine 6G dye laser that is synchronously pumped with the frequency-doubled output of a CW mode-locked Nd:YAG laser. These pulses are shortened to ~ 2 0 fs 0 in an optical fiber-grating compressor and amplified to =5 pJ at 1 kHz in a multipass rhodamine B dye amplifier which is pumped by the frequencydoubled output of a CW Q-switched Nd:YAG laser. The amplified output is split to derive both pump and gating pulses. The CW IR, which originates in a tunable diode laser (Laser Photonics, Analytics Division) or CO laser (Laser Photonics), is focused to =50-100 pm in a sample cell. The transmitted IR is frequency summed with the gating pulse in a LiIO:, crystal. A polarizer, prism and interference filters are used to isolate the upconverted light which is detected with a photomultiplier tube (PMT). A synchronous chopper alternately blocks the 1-kHz pump beam. Therefore the signal from the PMT contains a component at 1 kHz that corresponds to the averaged transmitted IR intensity ( T ) and a component a t 500 Hz that reveals the averaged differential transmitted IR intensity ( A T ) . The photocurrent is simultaneously demodulated in two independent lock-in amplifiers to recover the quantities ( T ) and ( A T ) ,which are used to compute the averaged differential IR absorbance ( P A ) . The diode laser wavelength is tunable by adjusting the temperature and/or current settings. The spectral region 1795-21 50 cm-l is covered with three diode lasers. Interchange among the diodes is readily accomplished in a few minutes by moving the cryostat that houses the diodes along a calibrated rail and adjusting the collimating off-axis parabolic reflector. While the diodes are capable of single-mode operation with a linewidth of
