State-to-state correlated study of CD3I photodissociation at 266 and 304 nm

High-resolution photofragment translational spectroscopy is used in this work to measure the translational and internal energy distributions in the CD3 and iodine fragments produced from the photodissociation of CD3I at 266 and 304 nm. Channel selected detection, via resonantly enhanced multiphoton ionization, combined with one-dimensional core sampling provides detailed information about vibrational state distributions of the CD3 fragments. The vibrational state distributions of CD3 fragments in the I*(2P12) channel have a propensity of nu2 ' umbrella bending mode with a maximum at nu2 ' = 1 for 266 nm photodissociation. For I*(2P12) channel at 304 nm photodissociation, vibrational state distributions of CD3 fragment have a maximum in the vibrational ground state. For the I(2P32) channel (1Q1 <-- 3Q(0+)), nu2 ' umbrella bending vibrational distribution is measured as the predominant vibrational mode but has a much broader distribution when compared to that of the I* channel. The vibrational state distributions of the CD3 fragment produced from the perpendicular transition, i.e., 3Q1, which was determined at 304 nm photodissociation, has a maximum at nu2 ' = 1. The curve crossing possibility between the 1Q1 and 3Q(0+) adiabatic potentials is determined as 0.19 for 266 and 0.85 for 304 nm. The trend in reaction dynamics in 266 and 304 nm photodissociation of CD3I is compared with theoretical calculations. A bond dissociation energy D0(C-I) = 56.60+/-0.5 kcal/mol was derived by applying laws of energy conservation.     

Bond-selective photodissociation of partially deuterated ammonia molecules: photodissociations of vibrationally excited NHD(2) in the 5nu(NH) state and NH(2)D in the 5nu(ND) state

Ultraviolet photodissociation of NHD(2) excited to the fourth overtone state of the NH stretching mode (5nu(NH)) and NH(2)D excited to that of the ND stretching mode (5nu(ND)) has been investigated by using a crossed laser and molecular beams method. Branching ratio between the NH and ND bond dissociations has been determined by utilizing a (2+1) resonance enhanced multiphoton ionization scheme of H and D atoms. For the photolysis of NHD(2) in the 5nu(NH) state, the NH dissociation cross section is 5.1+/-1.4 times as large as the ND dissociation cross section per bond. On the other hand, for the photolysis of NH(2)D in the 5nu(ND) state, the ratio of the NH dissociation cross section per bond to the ND dissociation cross section decreases to 0.68+/-0.16. In comparison with the branching ratios for the photolysis of vibrationally unexcited NH(2)D and NHD(2), the present results indicate that the excitation of the NH stretching mode enhances the NH dissociation with ca. two times larger NH/ND branching ratio, whereas the excitation of the ND stretching mode results in the preferential ND dissociation with ca. 3-4 times larger ND/NH branching ratio than that for the vibrational ground states. The mechanism of the bond-selective enhancement has been discussed in terms of the energetics and dynamics of wave packet.     

The photodissociation reaction dynamics of CF(3)I at 304 nm ((3)Q(0+), (1)Q(1)<--(3)Q(0+), and (3)Q(1))

The photodissociation of CF(3)I at 304 nm has been studied using long time-delayed core-sampling photofragment translational spectroscopy. Due to its capability of detecting the kinetic energy distribution of iodine fragments with high resolution, it is able to directly assign the vibrational state distribution of CF(3) fragments. The vibrational state distributions of CF(3) fragments in the I(*)((2)P(12)) channel, i.e., (3)Q(0+) state, have a propensity of the nu(2) (') umbrella mode with a maximum distribution at the vibrational ground state. For the I((2)P(32)) channel, i.e., (1)Q(1)<--(3)Q(0+), the excitation of the nu(2) (') umbrella mode accounts for the majority of the vibrational excitation of the CF(3) fragments. The 1 nu(1) (') (symmetric CF stretch) +nnu(2) (') combination modes, which are associated with the major progression of the nu(2) (') umbrella mode, are observed for the photodissociation of CF(3)I at the I channel, i.e., (3)Q(1) state. The bond dissociation energy of the CI bond of CF(3)I is determined to be D(0)(CF(3)-I)相似文献   

Quantum mechanical calculations for the H2O + hnu --> O(1D) + H2 photodissociation process

Quantum mechanical wavepacket calculations for the photodissociation of water in the second absorption band are presented. Using O + H2 Jacobi coordinates, partial cross sections for the O(1D) + H2 channel are calculated for different initial rotational states. Conical intersection and Renner-Teller effects are included. The branching ratios for the four accessible dissociation channels at 121.6 nm are in good agreement with experiment (J. Chem. Phys. 1982, 77, 2432). The calculations predict significant rotational and vibrational excitation of the H2 fragments. Photodissociation of ortho and para water produces predominantly, but not exclusively, ortho and para H2 fragments, respectively.     

On the conformational memory in the photodissociation of formic acid

The photodissociation of formic acid at 248 and 193 nm was investigated by classical trajectory and RRKM calculations using an interpolated potential energy surface, iteratively constructed using the B3LYP/aug-cc-pVDZ level of calculation. Several sampling schemes in the ground electronic state were employed to explore the possibility of conformational memory in formic acid. The CO/CO2 branching ratios obtained from trajectories initiated at the cis and at the trans conformers are almost identical to each other and in very good accordance with the RRKM results. In addition, when a specific initial excitation that simulates more rigorously the internal conversion process is used, the calculated branching ratio does not vary with respect to those obtained from cis and trans initializations. This result is at odds with the idea of conformational memory in the ground state proposed recently for the interpretation of the experimental results. It was also found that the calculated CO vibrational distributions after dissociation of the parent molecule at 248 nm are in agreement with the experimental available data.     

Anharmonic vibrational frequencies and vibrationally averaged structures and nuclear magnetic resonance parameters of FHF-

The anharmonic vibrational frequencies of FHF(-) were computed by the vibrational self-consistent-field, configuration-interaction, and second-order perturbation methods with a multiresolution composite potential energy surface generated by the electronic coupled-cluster method with various basis sets. Anharmonic vibrational averaging was performed for the bond length and nuclear magnetic resonance indirect spin-spin coupling constants, where the latter computed by the equation-of-motion coupled-cluster method. The calculations placed the vibrational frequencies at 580 (nu(1)), 1292 (nu(2)), 1313 (nu(3)), 1837 (nu(1) + nu(3)), and 1864 cm(-1) (nu(1) + nu(2)), the zero-point H-F bond length (r(0)) at 1.1539 A, the zero-point one-bond spin-spin coupling constant [(1)J(0)(HF)] at 124 Hz, and the bond dissociation energy (D(0)) at 43.3 kcal/mol. They agreed excellently with the corresponding experimental values: nu(1) = 583 cm(-1), nu(2) = 1286 cm(-1), nu(3) = 1331 cm(-1), nu(1) + nu(3) = 1849 cm(-1), nu(1) + nu(2) = 1858 cm(-1), r(0) = 1.1522 A, (1)J(0)(HF) = 124+/-3 Hz, and D(0) = 44.4+/-1.6 kcal/mol. The vibrationally averaged bond lengths matched closely the experimental values of five excited vibrational states, furnishing a highly dependable basis for correct band assignments. An adiabatic separation of high- (nu(3)) and low-frequency (nu(1)) stretching modes was examined and found to explain semiquantitatively the appearance of a nu(1) progression on nu(3). Our calculations predicted a value of 186 Hz for experimentally inaccessible (2)J(0)(FF).     

Photodissociation dynamics of the methyl radical at 212.5 nm: effect of parent internal excitation

Photodissociation dynamics of the CH3 radical at 212.5 nm has been investigated using the H atom Rydberg tagging time-of-flight method with a pure CH3 radical source generated by the photolysis of CH3I at 266 nm. Time-of-flight spectra of the H atom products from the photolysis of both cold and hot methyl radicals have been measured at different photolysis polarizations. Experimental results indicate that the photodissociation of the methyl radical in its ground vibrational state at 212.5 nm excitation occurs on a very fast time scale in comparison with its rotational period, indicating the CH3 dissociation at 212.5 nm occurs on the excited 3s Rydberg state surface. Experimental evidence also shows that the photodissociation of the methyl radical in the nu2 = 1 state of the umbrella mode at 212.5 nm excitation is characteristically different from that in the ground vibrational state.     

Mode-dependent enhancement of photodissociation and photoionization in a seven atom molecule

We report the first experimental demonstration of vibrational mode-dependent enhancement in photodissociation and photoionization of a seven atom molecule, methylamine (CH(3)NH(2)). The fundamental C-H stretches and the overtones or combinations of CH(3) bends were prepared via stimulated Raman excitation (SRE) prior to their 243.135 nm one-photon dissociation or two-photon ionization. The photodissociation or photoionization of the vibrationally excited molecules was achieved via 10 ns delayed or temporally overlapping SRE and UV pulses, respectively. It is shown that bending modes are more effective than stretches in promoting photodissociation and photoionization, since their UV excitation is favored by larger Franck Condon factors. This behavior provides clear evidence for vibrational mode-dependence in a relatively large molecule with a torsional degree of freedom, indicating that these modes survive intramolecular vibrational redistribution on a time scale considerably longer than hitherto inferred from previous studies.     

Quantum theory of photodissociation of polyatomic molecules: Application to HCN

A theory of direct collinear photodissociation is presented wherein the correct and different normal modes appropriate to the initial molecular state and the photofragments are employed. This remedies a serious deficiency in previous theories which assume that the reaction coordinates for the photodissociation is also a normal mode in the initial electronic state and that the remaining normal modes are the same for both. The theory provides an analytical expression for the vibrational energy distribution of the photofragments, and provides simple criteria for the occurrence of population inversions. Calculated vibrational distributions for HCN photodissociation are in agreement with experiment.     

Vibrational predissociation dynamics of methane-Ar: an ab initio approach

We calculated the cross sections for vibrational predissociation of methane-Ar induced by excitation of the methane nu 3 mode with the aid of an ab initio CH4-Ar potential depending explicitly on the nu 3 and nu 1 normal coordinates of the CH4 monomer. We found that dissociation into CH4 fragments excited in the nu 1 mode, a V-->V' process with very low kinetic energy release, strongly dominates over direct dissociation into Ar and ground state CH4, and is responsible for the line broadening observed experimentally. The (observed and calculated) strong variation of the line widths for the Van der Waals levels excited in combination with the nu 3 mode (giving states of A, F and E symmetry) is related to the opening up of appropriate nu 1 dissociation channels and the occurrence of rotational resonances in the nu 1 continuum in the energy range of the quasi-bound nu 3 levels.     

Relativistic multireference calculation of photodissociation of o-, m-, and p-bromofluorobenzene

Quantum chemical calculations with relativistic effects were performed on the photodissociation of o-, m-, and p-bromofluorobenzene (o-, m-, and p-BrFPh) at 266 nm. The method of multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction was employed to calculate the potential energy curves for the ground and low-lying excited states of o-, m-, and p-BrFPh along their photodissociation reaction coordinates. The dissociation mechanisms with products of Br((2)P(3∕2)) and Br(?)((2)P(1∕2)) states were clarified with the computed potential energy curves and the surface crossings. The current calculations augmented previous theoretical investigations by including relativistic effects and resolved some differences of experimental assignment regarding the dissociation channels of o-, m-, and p-BrFPh.     

Near-UV photolysis of substituted phenols, I: 4-fluoro-, 4-chloro- and 4-bromophenol

The experimental techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance-enhanced multiphoton ionisation time-of-flight spectroscopy have been used to investigate the dynamics of H atom loss processes from gas phase 4-fluorophenol (4-FPhOH), 4-chlorophenol (4-ClPhOH) and 4-bromophenol (4-BrPhOH) molecules, following excitation at many wavelengths, lambda(phot), in the range between their respective S(1)-S(0) origins (284.768 nm, 287.265 nm and 287.409 nm) and 216 nm. Many of the Total Kinetic Energy Release (TKER) spectra obtained from photolysis of 4-FPhOH show structure, the analysis of which reveals striking parallels with that reported previously for photolysis of bare phenol (M. G. D. Nix, A. L. Devine, B. Cronin, R. N. Dixon and M. N. R. Ashfold, J. Chem. Phys., 2006, 125, 133318). The data demonstrates the importance of O-H bond fission, and that the resulting 4-FPhO co-fragments are formed in a select fraction of their available vibrational state density. All spectra recorded at lambda(phot)> or = 238 nm show a feature centred at TKER approximately 5500 cm(-1). These H atom fragments show no recoil anisotropy, and are rationalised in terms of initial S(1)<-- S(0) (pi* <--pi) excitation and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S(0)) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer to (and round) the Conical Intersection (CI) between the S(0) and S(2)((1)pi sigma*) Potential Energy Surfaces (PESs) at larger R(O-H), en route to H atoms and ground state 4-FPhO products. The vibrational energy disposal in the 4-FPhO products indicates that parent mode nu(16a) promotes non-adiabatic coupling at the S(0)/S(2) CI. Spectra recorded at lambda(phot)< or = 238 nm reveal a faster (but still isotropic) distribution of recoiling H atoms, centred at TKER approximately 12 000 cm(-1), attributable to H + 4-FPhO products formed when the optically excited (1)pi pi* molecules couple directly with the (1)pi sigma* PES. Parent mode nu(16b) is identified as the dominant coupling mode at the S(1)((1)pi pi*)/S(2)((1)pi sigma*) CI, and the resulting 4-FPhO radical co-fragments display progressions in nu(18b) (the C-O in-plane wagging mode) and nu(7a) (an in-plane ring breathing mode involving significant C-O stretching motion). Analysis of all structured TKER spectra yields a C-F bond dissociation energy: D(0)(H-OC(6)H(4)F) = 29 370 +/- 50 cm(-1). The photodissociation of 4-ClPhOH shows many similarities, though the 4-ClPhO products formed together with faster H atoms at shorter wavelengths (lambda(phot)< or = 238 nm, by coupling through the S(1)/S(2) CI) show activity in an alternative ring breathing mode (nu(19a) rather than nu(7a)). Spectral analysis yields D(0)(H-OC(6)H(4)Cl) = 29 520 +/- 50 cm(-1). H atom formation via O-H bond fission is (at best) a very minor channel in the photolysis of 4-BrPhOH at all wavelengths investigated. Time-dependent density functional theory calculations suggest that this low H atom yield is because of competition from the alternative C-Br bond fission channel, and that the analogous C-Cl bond fission may be responsible for the weakness of the one photon-induced H atom signals observed when photolysing 4-ClPhOH at longer wavelengths.     

Relative vibrational overtone intensity of cis-cis and trans-perp peroxynitrous acid

The vibrational overtone spectrum of HOONO is examined in the region of the 2 nu(OH) and 3 nu(OH) bands using action spectroscopy in conjunction with ab initio intensity calculations. The present measurements indicate that the oscillator strength associated with the higher energy trans-perp conformer of HOONO is stronger relative to the lower energy cis-cis conformer for both these vibrational overtone levels. Ab initio intensity calculations carried out at the QCISD level of theory suggest that this disparity in oscillator strength apparently arises from differences in the second derivative of the transition dipole moment function of the two isomers. The calculations indicate that the oscillator strength for the trans-perp isomer is approximately 5.4 times larger than that of the cis-cis isomer for the 2 nu(OH) band and approximately 2 times larger for 3 nu(OH) band. The band positions and intensities predicted by the calculations are used to aid in the assignment of features in the experimental action spectra associated with the OH stretching overtones of HOONO. The observed relative intensities in the experimental action spectra when normalized to the calculated oscillator strengths appears to suggest that the concentration of the higher energy trans-perp isomer is comparable to the concentration of the cis-cis isomer in these room temperature experiments.     

OH-stretch vibrational spectroscopy of hydroxymethyl hydroperoxide

We report measurement and analysis of the photodissociation spectrum of hydroxymethyl hydroperoxide (HOCH(2)OOH) and its partially deuterated analogue, HOCD(2)OOH, in the OH-stretching region. Spectra are obtained by Fourier transform infrared spectroscopy in the 1nu(OH) and 2nu(OH) regions, and by laser induced fluorescence detection of the OH fragment produced from dissociation of HOCH(2)OOH initiated by excitation of the 4nu(OH) and 5nu(OH) overtone regions (action spectroscopy). A one-dimensional local-mode model of each OH chromophore is used with ab initio calculated OH-stretching potential energy and dipole moment curves at the coupled-cluster level of theory. Major features in the observed absorption and photodissociation spectra are explained by our local-mode model. In the 4nu(OH) region, explanation of the photodissocation spectrum requires a nonuniform quantum yield, which is estimated by assuming statistical energy distribution in the excited state. Based on the estimated dissociation threshold, overtone photodissociation is not expected to significantly influence the atmospheric lifetime of hydroxymethyl hydroperoxide.     

The photodissociation of ozone in the Hartley band: a theoretical analysis

Three-dimensional diabatic potential energy surfaces for the lowest four electronic states of ozone with 1A' symmetry-termed X, A, B, and R-are constructed from electronic structure calculations. The diabatization is performed by reassigning corresponding energy points. Although approximate, these diabatic potential energy surfaces allow one to study the uv photodissociation of ozone on a level of theory not possible before. In the present work photoexcitation in the Hartley band and subsequent dissociation into the singlet channel, O3X+hnu-->O(1D)+O2(a 1Deltag), are investigated by means of quantum mechanical and classical trajectory calculations using the diabatic potential energy surface of the B state. The calculated low-resolution absorption spectrum as well as the vibrational and rotational state distributions of O2(a 1Deltag) are in good agreement with available experimental results.     

Intracluster stereochemistry in van der Waals complexes: steric effects in ultraviolet photodissociation of state-selected Ar-HOD/H2O

High-resolution IR-UV multiple resonance methods are employed to elucidate the photodissociation dynamics of quantum state-selected Ar-HOD and Ar-H(2)O van der Waals clusters. A single mode pulsed OPO operating in the region of the OH second overtone is used to prepare individual rovibrational states that are selectively photodissociated at specific excimer wavelengths. Subsequent fluorescence excitation of the resulting OH (OD) fragments yields dynamical information on the photofragmentation event and any resulting intracluster collisions. This technique is used to characterize spectroscopically the Pi(1(01)), nu(OH)=3<--Sigma(0(00)), v(OH)=0 overtone band of the Ar-HOD complex with an origin at 10648.27 cm(-1). The effects of Ar complexation on the dissociation dynamics are inferred by comparison of the OD photofragment quantum state distributions resulting from dissociation of single rovibrational states of the complex with those from isolated HOD photodissociation. The important role played by the initial internal state of the complex is demonstrated by comparison of the current Ar-HOD data with previously published results for the Ar-H(2)O Sigma(0(00))[03(-)> state. We interpret the dramatic differences in the dynamics of the two systems as manifestations of the nodal structure of the vibrational state in the parent complex and the way in which it governs the collision probability between the Ar atom and the escaping photofragments.     

Cis-cis and trans-perp HOONO: action spectroscopy and isomerization kinetics

The weakly bound HOONO product of the OH+NO2+M reaction is studied using the vibrational predissociation that follows excitation of the first OH overtone (2nu1). We observe formation of both cis-cis and trans-perp conformers of HOONO. The trans-perp HOONO 2nu1 band is observed under thermal (223-238 K) conditions at 6971 cm(-1). We assign the previously published (warmer temperature) HOONO spectrum to the 2nu1 band at 6365 cm(-1) and 2nu1-containing combination bands of the cis-cis conformer of HOONO. The band shape of the trans-perp HOONO spectrum is in excellent agreement with the predicted rotational contour based on previous experimental and theoretical results, but the apparent origin of the cis-cis HOONO spectrum at 6365 cm(-1) is featureless and significantly broader, suggesting more rapid intramolecular vibrational redistribution or predissociation in the latter isomer. The thermally less stable trans-perp HOONO isomerizes rapidly to cis-cis HOONO with an experimentally determined lifetime of 39 ms at 233 K at 13 hPa (in a buffer gas of predominantly Ar). The temperature dependence of the trans-perp HOONO lifetime in the range 223-238 K yields an isomerization barrier of 33+/-12 kJ/mol. New ab initio calculations of the structure and vibrational mode frequencies of the transition state perp-perp HOONO are performed using the coupled cluster singles and doubles with perturbative triples [CCSD(T)] model, using a correlation consistent polarized triple zeta basis set (cc-pVTZ). The energetics of cis-cis, trans-perp, and perp-perp HOONO are also calculated at this level [CCSD(T)/cc-pVTZ] and with a quadruple zeta basis set using the structure determined at the triple zeta basis set [CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ]. These calculations predict that the anti form of perp-perp HOONO has an energy of DeltaE0=42.4 kJ/mol above trans-perp HOONO, corresponding to an activation enthalpy of DeltaH298 (double dagger 0)=41.1 kJ/mol. These results are in good agreement with statistical simulations based on a model developed by Golden, Barker, and Lohr. The simulated isomerization rates match the observed decay rates when modeled with a trans-perp to cis-cis HOONO isomerization barrier of 40.8 kJ/mol and a strong collision model. The quantum yield of cis-cis HOONO dissociation to OH and NO2 is also calculated as a function of photon excitation energy in the range 3500-7500 cm(-1), assuming D0=83 kJ/mol. The quantum yield is predicted to vary from 0.15 to 1 over the observed spectrum at 298 K, leading to band intensities in the action spectrum that are highly temperature dependent; however, the observed relative band strengths in the cis-cis HOONO spectrum do not change substantially with temperature over the range 193-273 K. Semiempirical calculations of the oscillator strengths for 2nu1(cis-cis HOONO) and 2nu1(trans-perp HOONO) are performed using (1) a one-dimensional anharmonic model and (2) a Morse oscillator model for the OH stretch, and ab initio dipole moment functions calculated using Becke, Lee, Yang, and Parr density functional theory (B3LYP), M?ller-Plesset pertubation theory truncated at the second and third order (MP2 and MP3), and quadratic configuration interaction theory using single and double excitations (QCISD). The QCISD level calculated ratio of 2nu1 oscillator strengths of trans-perp to cis-cis HOONO is 3.7:1. The observed intensities indicate that the concentration of trans-perp HOONO early in the OH+NO2 reaction is significantly greater than predicted by a Boltzmann distribution, consistent with statistical predictions of high initial yields of trans-perp HOONO from the OH+NO2+M reaction. In the atmosphere, trans-perp HOONO will isomerize nearly instantaneously to cis-cis HOONO. Loss of HOONO via photodissociation in the near-IR limits the lifetime of cis-cis HOONO during daylight to less than 45 h, other loss mechanisms will reduce the lifetime further.     

State-to-state reaction dynamics of CH3I photodissociation at 304 nm

The detailed reaction dynamics of CH(3)I photodissociation at 304 nm were studied by using high-resolution long time-delayed core-sampling photofragment translation spectroscopy. The vibrational state distributions of the photofragment, i.e., CH(3), are directly resolved due to the high kinetic resolution of this experiment for the first time. CH(3) radicals produced from I((3)Q(0+)), I((1)Q(1) <--( 3)Q(0+)), and I((3)Q(1)) channels are populated in different vibrational state distributions. The I((3)Q(0+)) and I((3)Q(1)) channels show only progressions in the nu2'(a2") umbrella bending mode, and the I((1)Q(1) <-- (3)Q(0+)) channel shows both progression in the nu2' umbrella bending mode and a small amount of excitation in the nu1'(a1') C-H stretching mode. The photodissociation processes from the vibrational hot band of CH(3)I (upsilon3 = 1, upsilon3 = 2) were also detected, primarily because of the absorption probability from the vibrational excited states, i.e., hot bands are relatively enhanced. Photofragments from the hot bands of CH(3)I show a cold vibrational distribution compared to that from the vibrational ground state of CH(3)I. The I* quantum yield and the curve crossing possibility were also studied for the ground vibrational state of CH(3)I. The potential energy at the curve crossing point was calculated to be 32 790 cm(-1) by using the one-dimensional Landau-Zener model.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.