Volume 105, Issue 18, 08 November 1996
Index of content:

Measurement of the ν_{3} fundamental transition moment and vibrational relaxation rates of the CD_{3} radical
View Description Hide DescriptionDiode laserabsorption spectroscopy was used to measure the transition dipole moment for the ν_{3} (degenerate asymmetric stretch) fundamental vibration of CD_{3} produced by the 193‐nm photodissociation of acetone‐d _{6}. The time evolution of the ground state absorption was used to measure the vibrational relaxation rates from the manifold of excited vibrational states to the ground state of CD_{3} following collisions with various bath gases. The transition dipole moment was determined to be 0.030±0.002 D and the vibrational relaxation coefficients were determined to be (2.5±0.2)×10^{−13} for argon, (2.6±0.2)×10^{−13} for helium, (3.15±0.2)×10^{−13} for nitrogen, and (4.3±0.5)×10^{−12} for acetone‐d _{6} in units of cm^{3} molecules^{−1} s^{−1}. Results are compared to literature values for CH_{3} and the mechanism of the relaxation is discussed.

Dissociative electron attachment and intramolecular electron transfer in linear haloalkenes
View Description Hide DescriptionIntramolecular electron transfer through alkyl chains has been investigated by measuring the cross section for halide detachment following resonant π* electron capture in linear n‐halo‐1‐alkenes, for halo = chloro, bromo and n=2–6. The magnitude of the cross section decreases with increasing chain length for all the haloalkenes, with the exception of the halopropenes, but at a considerably faster rate for the chloro than for the bromo compounds. The decrease in cross section for the chloroalkenes occurs at a rate consistent with the decrease in electron‐tunneling rates in hydrocarbons with through‐bond interactions. For the bromoalkenes it appears that σ*–π* coupling is quite strong and thus the results are not consistent with an electron transfer interpretation. Excluding the propenes, the energy of the cross section maximum is essentially constant for the chloralkenes while it decreases with chain length for the bromoalkenes. Hartree–Fock calculations have been used to determine the equilibrium geometries of various conformers of the n‐halo‐1‐alkenes for n=2–5. The 4‐ and 5‐bromo‐1‐alkenes show considerably smaller conformational energy differences than do the chloro compounds. Electron attachment energies have been calculated for the most stable conformers at Hartree–Fock, density functional, and Mo/ller–Plesset second order levels. Trends in calculated attachment energies parallel experimental trends in the energies of dissociative attachment maxima only for the Mo/ller–Plesett second order calculations. At the Hartree–Fock level the singly occupied molecular orbitals of the haloalkene radical anions show a somewhat greater admixture of C–halogen σ* and C=Cπ* character in the bromo compounds than in the chloroalkenes, but the distinct difference in σ*–π* coupling in the bromoalkenes compared to the chloroalkenes is represented accurately only in the calculations that include electron correlation.

IR–rf double resonance studies of dipole moments in the ν_{1}+ν_{4} and ν_{1}+ν_{5} states of acetylene‐d
View Description Hide DescriptionInfrared laser–radio frequency double resonancespectroscopy has been used to observe transitions across J=1 l‐type doublets in vibrational states of monodeuterated acetylene, HCCD, that have both the C‐H stretch and one degenerate bending mode excited. Electric dipole moments and l doublings have been measured for the ν_{1}+ν_{4} and ν_{1}+ν_{5} states, giving: μ_{14}=−0.046 44(3) D, q _{l(14)}=132.289(6) MHz, μ_{15}=+0.031 26(2) D, and q _{l(15)}=107.263(4) MHz.

Observation of strong hidden lines in the infrared spectrum of the CO–He complex
View Description Hide DescriptionIn the only previous observation of spectra of the weakly bound complex CO–He [C. E. Chuaqui, R. J. Le Roy, and A. R. W. McKellar, J. Chem. Phys. 101, 39 (1994)], only 6 out of the 21 strongest transitions were observed, and much of the analysis therefore relied on relatively weak transitions. These strong ‘‘hidden’’ transitions are located very close to, and were obscured by, pressure‐broadened transitions of the CO monomer. In the present paper, the measurement of all but one of the 15 hidden transitions has been achieved by using a sensitive tunable diode laser probe which allows much lower sample pressures. The results fully confirm the previous analysis, and provide additional precise data on the bound energy levels of this fundamental molecular system, especially for the CO–^{3}He isotope.

Investigation of molecular structure in solids by two‐dimensional NMR exchange spectroscopy with magic angle spinning
View Description Hide DescriptionAn approach to the investigation of molecular structures in disordered solids, using two‐dimensional (2D) nuclear magnetic resonance(NMR) exchange spectroscopy with magic angle spinning (MAS), is described. This approach permits the determination of the relative orientation of two isotopically labeled chemical groups within a molecule in an unoriented sample, thus placing strong constraints on the molecular conformation. Structural information is contained in the amplitudes of crosspeaks in rotor‐synchronized 2D MAS exchange spectra that connect spinning sideband lines of the two labeled sites. The theory for calculating the amplitudes of spinning sideband crosspeaks in 2D MAS exchange spectra, in the limit of complete magnetization exchange between the labeled sites, is presented in detail. A new technique that enhances the sensitivity of 2D MAS exchange spectra to molecular structure, called orientationally weighted 2D MAS exchange spectroscopy, is introduced. Symmetry principles that underlie the construction of pulse sequences for orientationally weighted 2D MAS exchange spectroscopy are explained. Experimental demonstrations of the utility of 2D MAS exchange spectroscopy in structural investigations of peptide and protein backbone conformations are carried out on a model ^{13}C‐labeled tripeptide, L‐alanylglycylglycine. The dihedral angles φ and ψ that characterize the peptide backbone conformation at Gly‐2 are obtained accurately from the orientationally weighted and unweighted 2D ^{13}C NMR exchange spectra.

Jahn–Teller effect in the ground and excited states of MnO^{2−} _{4} doped into Cs_{2}SO_{4}
View Description Hide DescriptionThe polarized low‐temperature absorption spectra of the 3d ^{1} ion MnO^{2−} _{4} in the Cs_{2}SO_{4} host consist of a very weak, highly‐structured band in the near‐infrared (NIR) region corresponding to the ^{2} E→^{2} T _{2}(d→d) transition and a series of intense ligand‐to‐metal charge transfer (LMCT) excitations above 16 000 cm^{−1}. As a result of the low‐symmetry crystal‐field (CF) potential in Cs_{2}SO_{4} the ^{2} T _{2} ligand‐field (LF) state of MnO^{2−} _{4} is split into its three orbital components at 10 557, 10 848, and 10 858 cm^{−1} above the ground state. The lowest‐energy component serves as initial state for broadband luminescence to the ^{2} Eground state, exhibiting unusually well‐resolved fine structure at 15 K. The orbital splitting of ^{2} E is 969 cm^{−1} and thus larger by more than 1 order of magnitude and of opposite sign compared to the result of a ligand‐field calculation within the angular‐overlap model (AOM). This discrepancy is explained with the large contribution of the second‐nearest neighbor Cs^{+} ions to the CF potential of MnO^{2−} _{4} in the Cs_{2}SO_{4} host lattice. The vibrational progressions in the ^{2} E↔^{2} T _{2}absorption and luminescencespectra are dominated by O‐Mn‐O bending modes. This is the result of a weak E⊗e and a stronger T _{2}⊗e Jahn–Teller (JT) effect in the ground and excited LF states, respectively. The observed vibronic levels in the luminescencespectrum are fitted with a single‐mode E⊗eJT Hamiltonian with an additional term representing the noncubic CF potential in Cs_{2}SO_{4}. The JT effect in the ^{2} T _{2} LF state causes a large displacement of the emitting level along the two coordinates of the e mode and thus substantially affects the intensity distribution in the luminescencespectrum. The fitted linear and quadratic vibronic constants for the ^{2} Eground state are 91 and 12 cm^{−1}, respectively, and for the ^{2} T _{2}excited state the linear coupling constant is −790 cm^{−1}. The corresponding JT stabilization energies are 14 and 925 cm^{−1} for ^{2} E and ^{2} T _{2}, respectively.

Structure and vibrational dynamics of aniline and aniline–Ar from high resolution electronic spectroscopy in the gas phase
View Description Hide DescriptionRotationally resolved S _{1}←S _{0} electronic spectra of aniline and its single atom van der Waals complex with argon (An–Ar) have been observed. Analysis of these spectra leads to a determination of the vibrationally averaged structures of the bare molecule and the complex in the two electronic states. Aniline itself is pyramidally distorted at the NH_{2} group in the S _{0} state. Attachment of the Ar atom on the side of the ring opposite the two N–H bonds converts the symmetric double well along the inversion coordinate into an asymmetric one, in the ground state. The excited state is quasiplanar along this coordinate. Analyses of the spectra of An–Ar at higher energies in the S _{1} state provide a probe of the vibrational predissociation (VP) behavior of the complex. We observe in these spectra line broadenings and spectral perturbations from which the important role of intra–intermolecular mode mixing (i.e., IVR) in promoting the VP process is elucidated.

Photophysics of size‐selected InP nanocrystals: Exciton recombination kinetics
View Description Hide DescriptionWe report here on the size‐dependent kinetics of exciton recombination in a III–V quantum dot system, InP. The measurements reported include various frequency dependent quantum yields as a function of temperature, frequency dependent luminescence decay curves, and time‐gated emission spectra. This data is fit to a three‐state quantum model which has been previously utilized to explain photophysical phenomena in II–VI quantum dots. The initial photoexcitation is assumed to place an electron in a (delocalized) bulk conduction band state. Activation barriers for trapping and detrapping of the electron to surface states, as well as activation barriers for surface‐state radiationless relaxation processes are measured as a function of particle size. The energy barrier to detrapping is found to be the major factor limiting room temperature band‐edge luminescence. This barrier increases with decreasing particle size. For 30 Å particles, this barrier is found to be greater than 6 kJ/mol—a barrier which is more than an order of magnitude larger than that previously found for 32 Å CdS nanocrystals.

Hyperfine quantum beats in the 209 nm fluorescence excitation spectrum of cyanogen
View Description Hide DescriptionSinglet–singlet, singlet–triplet, and polarization hyperfine fluorescencequantum beats are observed following 209 nm excitation of the 1^{1}4^{1} vibronic level of the Ã(^{1}Σ^{−} _{ u }) S _{1} singlet state. Singlet–singlet quantum beats arise from S _{1}–S _{0} vibronic coupling to states of the S _{0}manifold lying above the dissociation limit, but isolated from the dissociative continuum by a high barrier to predissociation.Measurements of fluorescence lifetimes and quantum beatspectral widths set an upper bound on this predissociation rate of approximately 10^{6} s^{−1}. The singlet–singlet quantum beats for the N=9 rotational state provide the first example of quantum beats arising from superposition states from two separate (I=0 and I=2) nuclear spin states. Singlet–triplet quantum beats for N=12 exhibit magnetically induced modulations that provide further support for a theoretical model previously used to describe similar modulations for the N=8, 12, and 18 rotational states of the 4^{1} vibronic level excited at 219 nm. Analysis of hyperfine polarization beats, associated with the reversible interchange of molecular polarization and nuclear spinpolarization, yields results in satisfactory agreement with the predictions of the perturbation theory formulation of Fano and Macek.

New measurements of the a ^{3} Σ^{+} _{ u } state of K_{2} and improved analysis of long‐range dispersion and exchange interactions between two K atoms
View Description Hide DescriptionResolved fluorescence from the K_{2} 4^{3} Σ^{+} _{ g } state to the a ^{3} Σ^{+} _{ u } state has been measured by the perturbation‐facilitated optical–optical double resonance (PFOODR) technique. Data have been fit to an improved set of molecular constants for the a ^{3} Σ^{+} _{ u } state. In particular, the new T _{ e } value for this state has been determined as 4197.935±0.047 cm^{−1}, nearly 1.8 cm^{−1} higher than previously reported. By combining the new results for the a ^{3} Σ^{+} _{ u } state and the recent results for the ground X ^{1} Σ^{+} _{ g } state [J. Chem. Phys. 103, 3350 (1995)], we report in this paper an improved analysis of long‐range dispersion and exchange interactions between two K atoms and of the X ^{1} Σ^{+} _{ g } and a ^{3} Σ^{+} _{ u } state dissociation energiesD _{ e } of 4450.674±0.072 cm^{−1} and 252.74±0.12 cm^{−1}, respectively.

Ab initio‐discrete variable representation calculation of vibrational energy levels
View Description Hide DescriptionA technique to calculate vibrational energy levels of a triatomic molecule without any explicit functional form for the potential energy surface (PES) is presented. The approach uses potential optimized discrete variable representation (DVR) to calculate the vibrational energy levels while ab initio electronic structure calculations are used to evaluate the potential energy at the nuclear configurations needed in the DVR calculation. The approach is called the ab initio‐discrete variable representation or ABI‐DVR technique. Example calculations for the water molecule are performed. Vibrational energy levels of H_{2} ^{16}O are calculated up to 14 000 cm^{−1} above the ground vibrational state within convergence better than 1 cm^{−1}. The potential energy is evaluated using GAUSSIAN 92 program suite. The 6‐311+G** Gaussian basis set is used and the electron correlation is taken into account by second‐order Möller–Plesset perturbation theory. The ABI‐DVR results are compared with results of calculations in which some analytic form for the PES is used to represent the ab initio calculated potential energies and some aspects of how to construct accurate analytic PESs are discussed.

Optical activity of electronically delocalized molecular aggregates: Nonlocal response formulation
View Description Hide DescriptionA unified description of circular dichroism and optical rotation in small optically active molecules, larger conjugated molecules, and molecular aggregates is developed using spatially nonlocal electric and magnetic optical response tensorsχ(r,r ^{ ′ },ω). Making use of the time dependent Hartree Fock equations, we express these tensors in terms of delocalized electronic oscillators. We avoid the commonly‐used long wavelength (dipole) approximation k⋅r≪1 and include the full multipolar form of the molecule–field interaction. The response of molecular aggregates is expressed in terms of monomer response functions. Intermolecular Coulomb interactions are rigorously taken into account thus eliminating the necessity to resort to the local field approximation or to a perturbative calculation of the aggregate wave functions. Applications to naphthalene dimers and trimers show significant corrections to the standard interacting point dipoles treatment.

Vibration–rotational spectroscopy of XH_{2} and XH_{3} type molecules near the local mode limit
View Description Hide DescriptionPresent work studies the effective rotational Hamiltonians and their vibration–rotational parameters for XH_{2} and XH_{3} type molecules near the local mode limit by including the diagonal matrix elements of coordinate operators when the bond anharmonicity is significant. An improved ‘‘α relation’’ is given for the local mode limit by taking the anharmonic bond oscillatorwave function as the basis function. Then the rotational tunneling approach is extended to model the effect of nonzero interbond coupling for XH_{2} and XH_{3} type molecules, which provides a dynamical view of the rovibrational structure of the local mode states.

Microwave spectroscopy of the HCCS and DCCS radicals (X̃ ^{2}Π_{ i }) in excited vibronic states: A study of the Renner–Teller effect
View Description Hide DescriptionThe microwave spectra of the HCCS and DCCS radicals are studied in the frequency range of 160–400 GHz and the rotational transition series are assigned to several low‐lying vibronic states in the CCS or H(D)CC bending vibration. Analysis is carried out to obtain effective constants for respective vibronic states. The γ_{eff} constants for the vibronic μ/κ^{2}Σ states are found to be anomalous, in that the variation of the γ_{eff} constants in the same bending mode is large up to 3 GHz and the γ_{eff} value can reach to nearly twice the rotational constantsB _{ v }. This behavior cannot be understood by the current Renner–Teller theory. We have developed a theory to include cross vibronic interaction between two vibronic ^{2}Σ(v _{ t }=1) states in different bending modes. Since the difference of the vibrational quantum numbers for these states is Δ(v _{4}+v _{5})=0, the interaction has a much larger effect than the one considered by Petelin and Kiselev [Int. J. Quantum Chem. 6, 701 (1972)] for the vibronic states with Δ(v _{4}+v _{5})=±2. Calculation with the newly derived expressions for γ_{eff} reproduces the anomaly in HCCS when the Renner parameters are fixed at ε_{4}=−0.37 and ε_{5}=+0.10 from the ab initio calculation, and the parameter ε_{45} for the cross vibronic interaction is varied to be 0.4, a value which is obtained for the first time. The relative sign of the above ε_{4} and ε_{5} values is explicitly judged to be correct. In addition, the B _{eff} and the P‐doubling constants in the ^{2}Π_{ i } and ^{2}Δ_{ i } states are found to be effected by a higher‐order perturbation of the cross vibronic interaction.

Tunneling dynamics, symmetry, and far‐infrared spectrum of the rotating water trimer. I. Hamiltonian and qualitative model
View Description Hide DescriptionA Hamiltonian is derived for the rotatingwater trimer with three internal motions—the rotations of the monomers about their hydrogen bonds. We obtain an expression of the kinetic energy operator, which is a non‐trivial extension of earlier heuristic forms used for the non‐rotating trimer. The Coriolis coupling operator between the single‐axis monomer angular momenta and the overall trimer rotation is given for the first time. To analyze the effects of the tunneling and Coriolis splittings on the energy levels of the trimer, we introduced a qualitative model for the pseudo‐rotation and donor tunneling. By perturbation theory and application of the permutation‐inversion groups G _{6} and G _{48} we obtain algebraic expressions for the splittings due to pseudo‐rotation and donor tunneling, respectively. The pseudo‐rotation does not produce any internal angular momentum and does not yield first order Coriolis splitting, but in second order the Coriolis coupling lifts various degeneracies and gives rise to observable J‐dependent splittings. Donor tunneling splits every pseudo‐rotation level into a quartet and those levels in this quartet that belong to the three‐dimensional irreps of G _{48} into doublets. For J≳0 a rather complex pattern of larger (for the internal states with G _{6} labels k=±1 and ±2) and smaller (for the levels with k=0 and k=3) splittings is obtained, especially for the substates with K=1 which are Coriolis coupled to the K=0 substates. The results of calculations in the companion paper, together with the model introduced in the present paper, will be used to interpret all the tunneling splittings observed in high‐resolution spectra of (H_{2}O)_{3} and (D_{2}O)_{3}.

Tunneling dynamics, symmetry, and far‐infrared spectrum of the rotating water trimer. II. Calculations and experiments
View Description Hide DescriptionWith the Hamiltonian derived in the preceding paper and the ab initio potentials of T. Bürgi, S. Graf, S. Leutwyler, and W. Klopper [J. Chem. Phys. 103, 1077 (1995)] and of J. G. C. M. van Duijneveldt‐van de Rijdt and F. B. van Duijneveldt [Chem. Phys. Lett. 237, 560 (1995)], we calculate the pseudo‐rotation tunneling levels in a rotating water trimer. The internal motions are treated by a three‐dimensional discrete variable representation and the Coriolis coupling with the overall rotation is included. Also the effects of donor tunneling are included, by introducing semi‐empirical coupling matrix elements. New experimental data are presented for the c‐type band at 87.1 cm^{−1} in (H_{2}O)_{3}, which show that specific levels in the donor tunneling quartets of this band are further split into doublets. With the results of our quantitative calculations and the model of the preceding paper we can understand the mechanisms of all the splittings observed in the earlier high‐resolution spectra of (H_{2}O)_{3} and (D_{2}O)_{3}, as well as these new splittings, in terms of pseudo‐rotation tunneling, donor tunneling and Coriolis coupling. An unambiguous assignment is given of all the bands observed and analyzed. The ab initio potential of the Van Duijneveldts yields accurate energies of the lower pseudo‐rotation levels, the potential of Bürgi et al. performs better for the higher levels. With our analysis we can deduce from the spectra that donor tunneling involves inversion of the trimer.

Unimolecular decomposition of chemically activated deutero‐substituted ethanol molecules studied by infrared chemiluminescence from H_{2}O, HOD, and D_{2}O
View Description Hide DescriptionVibrationally excited H_{2}O, HOD, and D_{2}O molecules formed by unimolecular elimination from deutero‐substituted ethanol molecules C_{2}H_{5}OH*, C_{2}H_{5}OD*, CH_{2}DCH_{2}OH*, and CH_{2}DCH_{2}OD* with an excitation energy of about 100 kcal mol^{−1} were observed by infrared chemiluminescence in the 2400–3900 cm^{−1} range. The activated ethanol molecules were produced via the successive reactions H+CH_{2}ICH_{2}OH→HI+CH_{2}CH_{2}OH and H+CH_{2}CH_{2}OH→CH_{3}CH_{2}OH* in a fast flow reactor that was observed with a Fourier transform spectrometer. The vibrational distributions of the H_{2}O, HOD, and D_{2}O molecules were determined by computer simulation of the experimental spectra; the distributions decline with increasing vibrational energy giving 〈f _{ v }〉=0.15 and 〈f _{ v }〉=0.14 for H_{2}O and HOD from the decomposition of C_{2}H_{5}OH* and C_{2}H_{5}OD*, respectively. The vibrational energy in the bending mode of H_{2}O is comparable to the energy in the stretching modes. Comparison with the statistical vibrational distributions shows a substantial overpopulation of the bending levels and a preferential excitation of one O–H or O–D stretching quantum in HOD from C_{2}H_{5}OD or CH_{2}DCH_{2}OH, respectively, i.e., in the newly formed bond. Kinetic isotope effects of [H_{2}O]/[HOD]=3.6±0.8 and [HOD]/[D_{2}O]=3.1±0.8 were found for the two elimination pathways of CH_{2}DCH_{2}OH* and CH_{2}DCH_{2}OD*, respectively, which agree with calculated RRKM values of k _{H2O}/k _{HOD}=3.2 and k _{HOD}/k _{D2O}=2.7.

Importance of hindered rotations in the thermal dissociation of small unsaturated molecules: Classical formulation and application to HCN and HCCH
View Description Hide DescriptionA standard low‐pressure limit Rice–Ramsperber–Kassel–Marcus rate constant is shown to significantly underestimate, by factors of three or more, the measured thermal dissociation rates for HCCH and HCN if the correct value of the bond‐dissociation energy is used. An explanation for this discrepancy is sought by examining anharmonic effects due to isomerization. Classical expressions for the density of states and partition function are developed which include isomerization anharmonicity and can be substituted in the standard rate constant expression for corresponding harmonic terms. These expressions are then applied to HCN and HCCH. For HCN, the resulting expression can be compared both to experiment and to a previous quantum mechanical study using the same Hamiltonian form and potential for isomerization. The classical and quantum mechanical agreement is excellent. Good agreement with experiment is obtained with the consensus dissociation energy. For HCCH, electronic structure calculations are performed to produce the required potential for isomerization. With this potential, comparison between measured rate constants and those calculated with the consensus dissociation energy is also good. In both of these applications, adiabatic influences from the two stretching frequencies are argued to reduce the effective isomerization barrier and increase the effective mass of the rotation. Based on these detailed applications, an approximate, closed‐form multiplicative factor for the rate constant expression is derived. This expression can be regarded as a generalization of one‐dimensional hindered rotor formulas for the inherently multidimensional hindered rotors of isomerization. The expression is parametrized by the height of the hindered‐rotor barrier. With the correct barrier height, this expression reproduces the more detailed calculations on HCN and HCCH. Its application to other systems indicates that the kinetic importance of isomerization in olefins is a rather general effect, not relegated only to small molecules.

On the shattering of clusters by surface impact heating
View Description Hide DescriptionThe onset of a shattering regime when a supersonic cluster undergoes an ultrafast heating by its impact at a surface, proposed on the basis of an information theoretic analysis, has now been demonstrated experimentally for molecular clusters. It is emphasized that the sudden onset of shattering as a function of impact velocity is a robust result depending essentially only on the multitude of possible isomers of larger clusters. There is one underlying assumption of the information theoretic approach—namely that there is a rather rapid thermalization of the translational degrees of freedom of the impact heated cluster so that mean energy is the only energetic constraint. When this is not necessarily the case, e.g., for ionic clusters at lower energies, there will not be extensive fragmentation.

The HNCO heat of formation and the N–H and C–N bond enthalpies from initial state selected photodissociation
View Description Hide DescriptionWe measure upper limits for the bondenthalpies of the N–H and C–N bonds in HNCO by observation of photodissociation appearance thresholds for the NCO (X ^{2}Π) and NH (a ^{1}Δ) fragments from initially selected HNCO vibrational states. The upper limit of the dissociation energy of the H–N bond is D _{0}(H–NCO)≤109.6±0.4 kcal/mol and that of the N–C bond is D _{0} (HN–CO)≤122.1±0.3 kcal/mol. Observation of unrelaxed fragment quantum state distributions at fixed energies supports the bondenthalpymeasurement. The two appearance thresholds, together with known heats of formation of NH, NCO, H, and CO, provide two independent methods of calculating the HNCO heat of formation. Both methods give a value of ΔH_{ f 0 } ^{0} (HNCO)≥−27.7±1.1 kcal/mol. The consistency of the two methods for calculating ΔH_{ f 0 } ^{0} (HNCO) suggests that the actual bondenthalpies for the N–H and C–N bonds are close to the upper limits from the measurement.