Quantum monodromy in NCNCS - direct experimental confirmation

Since the first confirmation of quantum monodromy in NCNCS (B. P. Winnewisser et al., Report No. TH07 in 60th International Symposium on Molecular Spectroscopy, Columbus, OH, (2005) and B. P. Winnewisser et al., Phys. Rev. Lett., 2005, 95, 243002) we have continued to explore its implications for the quantum structure of molecules. To confirm quantum monodromy bending-vibrational + axial-rotational quantum energy level information is needed. This was not directly available from the pure a-type rotational transitions available in 2005. The confirmation of quantum monodromy therefore had to be based on the fitting of the Generalised SemiRigid Bender (GSRB) model to the experimental rotational data. The GSRB is a physically motivated model and was able to extract the required information based on the changes of the rotational energy level structure upon excitation of the bending vibration and of the axial rotation. These results were, in some sense, predictions. Our goal here was to obtain a fully experimental and unambigous confirmation of quantum monodromy in NCNCS. This involved a series of experimental campaigns at the Canadian Light Source (CLS) synchrotron. To coax the required information out of the masses of spectral data that had been obtained a variety of techniques had to be used. The result is that we can now confirm, without recourse to a theoretical model, the existence of quantum monodromy in the ν₇ bending mode of NCNCS. As a side benefit we also confirm the power of the GSRB model to extract the required information from the previously available data. The predictions previously provided by the GSRB were surprisingly accurate. Only a slight augmentation of the model was required to allow us to refit it including the new data, while maintaining the quality of the fitting for that data previously available. We also present a very basic introduction to the idea of monodromy and to how the GSRB was used.

A rigorous detection of interstellar CH3NCO: An important missing species in astrochemical networks

The recent analysis of the composition of the frozen surface of comet 67P/Churyumov-Gerasimenko has revealed a significant number of complex organic molecules. Methyl isocyanate (CH3NCO) is one of the more abundant species detected on the comet surface. In this work we report extensive characterization of its rotational spectrum resulting in a list of 1269 confidently assigned laboratory lines and its detection in space towards the Orion clouds where 399 lines of the molecule have been unambiguously identified. We find that the limited mm-wave laboratory data reported prior to our work require some revision. The abundance of CH3NCO in Orion is only a factor of ten below those of HNCO and CH3CN. Unlike the molecular abundances in the coma of comets, which correlate with those of warm molecular clouds, molecular abundances in the gas phase in Orion are only weakly correlated with those measured on the comet surface. We also compare our abundances with those derived recently for this molecule towards Sgr B2 (Halfen et al. 2015). A more accurate abundance of CH3NCO is provided for this cloud based on our extensive laboratory work.

Pursuit of quantum monodromy in the far-infrared and mid-infrared spectra of NCNCS using synchrotron radiation

Quantum monodromy has a dramatic and defining impact on all those physical properties of chain-molecules that depend on a large-amplitude bending coordinate, including in particular the distribution of the ro-vibrational energy levels. As revealed by its pure rotational (a-type) spectrum [B. P. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189] cyanogen iso-thiocyanate, NCNCS, is a particularly illuminating exemplar of quantum monodromy: it clearly shows the distinctive monodromy-induced dislocation of the ro-vibrational energy level pattern for its low-lying bending mode. This dislocation centers on a lattice defect in the energy vs. momentum map of the ro-vibrational levels at the top of the barrier to linearity, and represents an example of an excited state quantum phase transition [D. Larese and F. Iachello, J. Mol. Struct., 2011, 1006, 611-628]. To complete the data, so far limited to ΔJ = +1 transitions, we decided to measure the high-resolution far-infrared band of the large-amplitude bending vibration ν7, and, if possible, mid-infrared bands. This Perspectives article presents our ongoing progress towards this goal, beginning with the description of how to predict line positions and intensities of the a- and b-type bands of the large amplitude bending mode using the Generalized-SemiRigid-Bender (GSRB) Hamiltonian for NCNCS and ab initio dipole moment functions [B. P. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189]. We include background information about synchrotron physics to clarify the advantages and limitations of that radiation source for our experiments. Details of the chemical preparation and sample handling, leading to the realization that NCNCS is 50 kJ mol(-1) lower in energy than its isomer S(CN)2 [Z. Kisiel et al., J. Phys. Chem. A, 2013, 117, 13815-13824] are included. We present the far-infrared and mid-infrared spectrum of NCNCS obtained at the Canadian Light Source synchrotron, using the IFS 125HR Bruker Fourier transform spectrometer. Eight of the fundamental vibrational modes of NCNCS have now been observed at high resolution. Initial analyses of the data confirm band assignments and demonstrate the accuracy of the predictions.

The Millimeter Wave Spectrum of DCNO: An Example of Current Measurements in the Frequency Range from 60 to 350 GHz

An efficient system for preliminary data reduction is described which completes a recently developed data acquisition and reduction system for the measurement of millimeter wave absorption lines with the help of a dedicated computer. A simple method of automatically determining the absorption line centers is given.

Rotational transitions of DCNO, measured with the above system, are reported for the ground state and the first excited state of each of the two bending modes ν4 and ν5. The rotational and rotation-vibration constants obtained for these states are

B0 = 10,292.48340 (31) MHz, D0 = 3.5418 (10) kHz,

Bν4 = 10,306.00780 (45) MHz, Dν4 = 3.6409 (22) kHz,

Bν5 = 10,338.65942 (32) MHz, Dν5 = 3.6208 (16) kHz.

The l-type doubling constants q.t 0) and qt (1) agree with the values obtained previously from direct l-type doubling transitions.

The Molecular Structures of HNC and HCN Derived from the Eight Stable Isotopic Species

Ground vibrational state rotational transitions have been measured for the following isotopic species of HNC: H15NC, H15N13C, DN13C, D15NC and D15N13C. Similar transitions have also been measured for H13C15N and D13C15N. These measurements complete the set of rotational constants for the eight stable isotopic species of the two molecules. Molecular structures are calculated from these data in several ways and are compared with the results of recent ab initio calculations.

New Measurements of the Infrared Spectrum of H2S2 and D2S2 and Evaluation of the Molecular Force Field

The infrared spectrum of H2S2 has been reviewed in the range 4000 to 250 cm-1 and the spectrum of D2S2 has been measured for the first time. No change in the basic assignment of the fundamental bands of H2S2 has been made, although the stretching fundamentals and the combination bands can be more precisely assigned on the basis of the new information.

With the six fundamental frequencies of H2S2 and the two asymmetric fundamental frequencies of D2S2 , calculations were made to determine as fully as possible the general valence force field. It was found that only the diagonal force constants could be determined on the basis of the available data:

FSH = 4.08 - 4.09 md/Å, Fa = 0.83 - 0.85 md A/rad2,

FSS = 2.52 -2.62 md/Å, Ft = 0.0926 md Å/rad2.

Mikrowellenspektrum, Struktur und l-Typ-Dublett-Aufspaltung der HCNO (Knallsäure)

The microwave spectra of unstable, gaseous fulminic acid H12C14N16O and five of its isotopically substituted species have been studied in the frequency range from 10 to 46 GHz. The spectrum of molecules in the ground vibrational state established the linearity of the chain HCNO. The following rotational constants B0 for the ground state were obtained:

B0 (H12C14N16O) = 11 469.04 MHz, B0 (H12C14N17O) = 11 151.69 MHz,

B0 (D12C14N16O) = 10 292.51 MHz, B0 (H12C14N18O) = 10 865.34 MHz,

B0 (H13C14N16O) = 11 091.57 MHz, B0 (D13C14N16O) = 10 011.18 MHz.

From these a combined r8- and r0-structure has been evaluated: r(C—H) = (1.027±0.001) A, r(C—N) = (1.161 ± 0.015) A, r(N—O) = (1.207±0.015) A.

The rather large uncertainties in the C—N- and N—O-distances are due to the proximity of the N-atom to the center of gravity.

It appears to be the first time that, in the same molecule, two different ι-type doublets (Δl=0, AJ= + 1) and their corresponding series of /-type doubling transitions (Al =2, ΔJ = 0), arising from the two degenerate bending modes (υ4=1 and v5=1), have been observed.

The analysis of the two ι-type doubling series revealed a marked dependence of the /-type doubling constants q4 and q5 on higher powers of the angular momentum J. This J-dependence was found to be substantially different for the two vibrational modes.

The doubling constants given in MHz are

q4=23.6722 - (0.6139·10-3) J(J+1) + (0.1417·10-6) [J(J+1)]2-(0199·10-10) [J(J+1)]3,

q5=34.6391 - (0.1623·10-3) J(J+1) + (1.00·10-9) [J(J+1)]2.

The molecular dipole moment was determined from Stark-effect measurements on the J=0 → 1 transition in the ground state and found to be (3.06 ± 0.15) Debye. The nuclear quadrupole coupling constants for the 14N- and 170-nuclei can be given as

e q Q (17O) = — (12.31 ± 0.12) MHz and | e q Q (14N) | ≦ 0.3 MHz.

A Parameter to Quantify Molecular Quasilinearity

A simple parameter is proposed for expressing the quasilinearity of molecules in a quantitative way. The parameter γ0 is defined on the basis of a discussion of the separation of variables in the Born-Oppenheimer approximation. The parameter γ0 assumes the value +1 for bent molecules and -1 for linear molecules. The range from -1 to +1 traces the transition of a vibrational degree of freedom in the linear configuration of a polyatomic molecule into a rotational degree of freedom in the bent configuration. The sequence of molecules along this parameter is shown and model calculations are carried out to further illustrate the physical interpretation of the parameter.

Detection of the Millimeter Wave Spectrum of Hydrogen Isocyanide, HNC1,2

The rotational spectra of three isotopic species of HNC have been observed in the millimeter wave region. In the case of the parent species, H14N12C, our measurement of the J = 1←0 transition unequivocally confirms the assignment of the U90.7 interstellar line to HNC. For the parent species the molecular constants obtained are B0 = 45332.005(40) MHz and D0 = 0.1019(50) MHz. Structural parameters derived from the ground state rotational constants of this linear molecule are r(H-N) = 0.987 (3) Å and r(N-C) =1.171 (4) Å.

Analysis of the Microwave Spectrum of Tricarbon Oxide Sulfide, 0 = C = C = C = S, in Highly Excited Bending States

In The microwave spectrum of tricarbon oxide sulphide (3-thioxo-l,2-propadiene-l-one), 0=C= C=C=S, has been measured in the frequency range from 8 to 40 GHz and includes vibrational satellite lines arising from the vibrational manifold of the lowest-lying doubly-degenerate bending mode i>7. The method of analysis followed the theory of rotation-vibrational interaction as devel-oped by Nielsen and Amat. The spectroscopic constants B v and D v as well as several vibration-rotational constants for each vibrational state vy = 0 to 7, = 1 and vq = 1 were determined with the aid of a newly written least squares program. The program structure relies on the cor-relation of the symmetry classification of energy levels in linear and bent molecules, and follows closely the analysis of asymmetric rotor spectra. From the «»vibrational spectrum of C3OS in excited states of vy up to t'7 — 7 and 1 — 6 the sign and magnitude of the effective vibrational anharmonicity constant xi t i t were determined. The interpretation of these results yields the infor-mation that in C3OS the potential function describing the two-dimensional oscillator of the vj bending mode is very harmonic and does not contain a perturbing hump. The dynamic behaviour of C3OS, a classical example of a linear molecule, is thus in sharp contrast to the quasilinear be-haviour of C3O2 •

Strukturbestimmung von Schwefeldioxid mit Hilfe der Elektronenbeugung an Gasen

Die Struktur des Schwefeldioxids wurde durch Elektronenbeugungsmessungen bei ca. 45 kV Beschleunigungsspannung ermittelt. Die sich ergebenden Strukturparameter wurden mit den Ergebnissen mikrowellenspektroskopischer Messungen verglichen, nachdem der S—O-Abstand mit Hilfe des ebenfalls bestimmten Anharmonizitätskoeffizienten auf den re-Wert korrigiert worden war. Die Kraftkonstante der S—O-Bindung wurde aus der Anharmonizitätskonstanten berechnet und mit den Ergebnissen anderer Autoren verglichen.

Centrifugal Distortion Analysis of the Microwave and Millimeter Wave Spectra of Deuterated Ketenes1,2

Measurements of R and Q branch a-type pure rotational transitions in the frequency range from 8 GHz to 220 GHz are reported for ketene-d1 and ketene-d2. The microwave and millimeter wave transitions were analysed in terms of Watson’s reduced Hamiltonian, including the sextic terms HKJ and HJK. The values of the inertial defect and the τ defect obtained from the centrifugal distortion analysis are in accord with the planar model for the ketene molecule.

Ground State Centrifugal Distortion Constants of Trans-Acrolein, CH2 = CH — CHO from the Microwave and Millimeter Wave Rotational Spectra 1,2

The pure rotational spectrum of trans-acrolein in the ground vibrational state has been assigned in the frequency region from 8 GHz to 180 GHz. The measured absorption lines encompass a-type transitions from the qRK, qQ1, qQ2 branches and 6-type transitions from the rP0, rP1, rP2, rR0 brandies for values of J up to 23. The rotational constants have been refined and all quartic and sextic centrifugal distortion constants have been determined using Watson's reduced Hamiltonian. This information has been used to predict line positions of astrophysical interest to warrant the interstellar line search for trans-acrolein.

Comments on Quartic Centrifugal Distortion Constants for Asymmetric Top Molecules

Explicit relations between the spectroscopic constants, A͂ , B͂ , C͂, ⊿J,⊿JK, ⊿K, δJ,and δK,

obtained by the application of the various reduction procedures of the effective rotational Hamil­tonian are given in matrix formulation. Watson’s reduction procedures for the three different axis representations Ir, IIr, and IIIr have been considered as well as the reduction making R͂5 = 0, in­stead of R͂6 = 0.

The ambiguity in the determination of the τ’s arising from the use of planarity conditions is discussed in relation to the τ defect.

l-Type Doublet Transitions of the Rare Stable Isotopic Species of Hydrogen Cyanide, HCN

l-type doublet transitions for HCN molecules in the first excited bending state 0110 have been measured for the isotopic species H13C14N, H12C15N, H13C15N, D13C14N, D12C15N and D13C15N. The measurements improve and complete the set of available coupling constants of the vibrationrotation interaction in this bending mode. All known l-type coupling constants are collected and compared with those obtained from the l-type splitting of the rotational lines and those obtained from ab-initio calculation of the energy hypersurface.

Elektronenbeugungsmessungen zur Struktur des gasförmigen Disulfans

An electron diffraction investigation of unstable gaseous disulfane using the sector-microphotometer method has been carried out. The results of these measurements and the available microwave data together yield the following molecular parameters for H2S2:

d (S—H) = 1.352 ± 0.015 A, angle (HSS) = 91°57′, d(S—S) = 2.055 ± 0.001 A, dihedral angle = 90°37′ ± 3′.

Analysis of the FASSST rotational spectrum of NCNCS in view of quantum monodromy

Quantum monodromy has a strong impact on the ro-vibrational energy levels of chain molecules whose bending potential energy function has the form of the bottom of a champagne bottle (i.e. with a hump or punt) around the linear configuration. NCNCS, cyanogen iso-thiocyanate, is a particularly good example of such a molecule and clearly exhibits a distinctive monodromy-induced dislocation of the energy level pattern at the bending-rotation energy at the top of the potential energy hump. Indeed, NCNCS [B. P. Winnewisser et al., Phys. Rev. Lett. 2005, 95, 243002] and the water molecule [N. F. Zobov et al., Chem. Phys. Lett. 2005, 414, 193-197] were the first two molecules for which experimental confirmation of quantum monodromy was obtained. We used the fast scan sub-millimetre spectroscopic technique (FASSST) to extend the measurements and spectral analysis to pure rotational transitions (end-over-end) in bending vibrational states lying well above the monodromy point. The analysis of 9204 lines assigned to 7 vibrational states, presented here, shows that the topological properties of the bending potential function are mapped onto every aspect of the ro-vibrational energy levels involving excitation of the quasi-linear bending vibration. In order to model the large amplitude dynamics of such a molecular system, and also to achieve some insight beyond satisfactory parameters for reproducing the spectrum, we used the generalized semi-rigid bender (GSRB) Hamiltonian, which is described in some detail. This Hamiltonian provides a good description of the energy levels over the seven bending states observed, coming close to experimental accuracy. Due to high J values of the measured rotational transitions (J</= 116), the least squares fitting procedure was applied not directly to the measured frequencies, but to effective constants derived from fitting the transition frequencies to a set of polynomials in J(J + 1) yielding effective B(eff) and D(eff) constants. The GSRB wave functions are used to show that the expectation values of any quantity which varies with the large amplitude bending coordinate will also have monodromy-induced dislocations. This includes the electric dipole moment components. High level ab initio calculations not only provided the molecular equilibrium structure of NCNCS, but also the electric dipole moment components mu(a) and mu(b) as functions of the large-amplitude bending coordinate. Calculated expectation values of these quantities for individual ro-vibrational levels show the now recognizable monodromy pattern. Finally, a generalization of the quasi-linear parameter gamma(0) is suggested.

Experimental confirmation of quantum monodromy: the millimeter wave spectrum of cyanogen isothiocyanate NCNCS

We have made energy-momentum maps for the experimental end-over-end rotational energy and the two-dimensional bending vibrational energy, both of which confirm the dominating effects of nontrivial quantum monodromy in cyanogen isothiocyanate. Accidental resonances in the rotational spectra yield accurate intervals between bending states.

The Rotational Spectrum of OCCCS Revisited

The rotational spectrum of the linear chain molecule tricarbon oxide sulfide, OCCCS, was recorded at room temperature between 118 and 179 GHz using a new commercial millimeter-wave spectrometer. The observed transitions were analyzed together with hitherto unassigned transitions from measurements between 78 and 118 GHz. About 1900 rotational transitions of OCCCS in the vibrational ground state and in 13 excited bending states could be assigned, the latter including the eighth, ninth, and tenth excited state of the low-lying bending mode nu(7) as well as the second excited state of the bending mode nu(6). Besides the determination of effective constants for these newly identified vibrational states, our data also served to considerably improve the effective constants for some of the bending states already characterized in previous publications. Copyright 2000 Academic Press.

Simultaneous Rovibrational Analysis of the nu(2), nu(3), nu(5), and nu(6) Bands of H(3)(12)CF

A total of 4892 transition wavenumbers of the nu(2), nu(3), nu(5), and nu(6) fundamental bands of H(3)(12)CF and 1469 frequencies of the rotational transitions in the v(2) = 1, v(3) = 1, v(5) = 1, and v(6) = 1 vibrational states have been fitted simultaneously by taking explicitly into account various Coriolis interactions, l-type interactions, and alpha-resonance terms between and within the vibrational states. The standard deviation of the fit using 71 parameters was 8.10 x 10(-5) cm(-1) for the infrared data and 0.198 MHz for the rotational data. This study clearly shows that in order to obtain a really quantitative fit of the highly precise data of vibration-rotational and rotational spectroscopy, it is necessary to fit all the data simultaneously using a variational approach. The deperturbed values of the upper state rotational constants differ significantly from those obtained previously in fitting the nu(3) and nu(6) bands as isolated bands and the nu(2)/nu(5) bands by a variational approach. This is important for the precise determination of the equilibrium structure of methyl fluoride (J. Demaison, J. Breidung, W. Thiel, and D. Papousek, Struct. Chem., in press). Besides this, the results can be used in further studies of the reduced Hamiltonians for more complicated systems of interacting vibrational levels. Copyright 1999 Academic Press.

Climbing the Bending Vibrational Ladder in D13C15N by Hot Gas Emission Spectroscopy

Using a newly constructed Fourier transform emission apparatus, we have measured the threefold substituted HCN isotopomer, D13C15N, at 1370 K in the range from 450 to 700 cm-1. We could assign hot bands with upper states up to Kvl2 = 12(12). The assignments have been verified for states up to v2 = 5 by fitting with earlier room temperature absorption measurements of overtone and hot bands. The intensities are shown to be in qualitative agreement with the expected intensity pattern for such emission spectra. All the measurements for D13C15N have been combined in a single least-squares fit that includes approximately 2700 rovibrational lines which have a root-mean-square deviation on the order of 0.000 4 cm-1. The spectroscopic constants for the bending states v2 = 1, ellipsis,12 are reported, as well as for some combination states involving the other vibrational modes. We also give the spectroscopic constants of various states of DCN, D13C14N, and D12C15N which where obtained from room temperature absorption measurements. Copyright 1999 Academic Press.

Absolute Line Intensities for the nu6 Band of H2O2

The purpose of this work was to obtain reliable absolute intensities for the nu6 band of H2O2. It was undertaken because strong discrepancies exist between the different nu6 band intensities which are presently available in the literature (A. Perrin, A. Valentin, J.-M. Flaud, C. Camy-Peyret, L. Schriver, A. Schriver, and P. Arcas, J. Mol. Spectrosc. 1995. 171, 358), (R. May, J. Quant. Radiat. Transfer 1991. 45, 267), and (R. L. Sams, personal communication). The method which was chosen in the present work was to measure simultaneously the far-infrared absorptions and the nu6 absorptions of H2O2. Consequently, Fourier transform spectra of H2O2 were recorded at Giessen in a spectral range (370-1270 cm-1) which covers both the R branch of the torsion-rotation band and the P branch of the nu6 band which appear at low and high wavenumbers, respectively. From the low wavenumber data, the partial pressure of H2O2 present in the cell during the recording of the spectra was determined by calibrating the observed absorptions in the torsion-rotation band with intensities computed using the permanent H2O2 dipole moment measured by Stark effect (A. Perrin, J.-M. Flaud, C. Camy-Peyret, R. Schermaul, M. Winnewisser, J.-Y. Mandin, V. Dana, M. Badaoui, and J. Koput, J. Mol. Spectrosc. 1996. 176, 287-296) and [E. A. Cohen and H. M. Pickett, J. Mol. Spectrosc. 1981. 87, 582-583). In the high frequency range, this value of the partial pressure of H2O2 was used to measure absolute line intensities in the nu6 band. Finally, the line intensities in the nu6 band were fitted using the theoretical methods described in detail in our previous works. Using these new results on line intensities together with the line position parameters that we obtained previously, a new synthetic spectra of the nu6 band was generated, leading to a total band intensity of 0.185 x 10(-16) cm-1/(molecule.cm-2) at 296 K. It has to be pointed out that the new line intensities agree to within the experimental uncertainties with the individual line intensity measurements performed previously by May and by Sams. Copyright 1999 Academic Press.

Simultaneous Rovibrational Analysis of the nu2, nu3, nu5, and nu6 Bands of H313CF

A total of 6071 transition wavenumbers of the nu2, nu3, nu5, and nu6 fundamental bands of H313CF and 1689 frequencies of the rotational transitions in the v2 = 1, v3 = 1, v5 = 1, and v6 = 1 vibrational states have been fitted simultaneously by taking into account various Coriolis interactions, l-type interactions, and alpha-resonance terms between and within the vibrational states. The standard deviation of the fit using 75 parameters was final sigma = 7.63 x 10(-5) cm-1 for the infrared data and 0.150 MHz for the rotational data. The possibility of obtaining a quantitative fit of the very precise data of high-resolution Fourier transform and submillimeter-wave spectroscopy for a semirigid symmetric top molecule is discussed. Copyright 1998 Academic Press.

Assignment and Power Series Analysis of the FIR Fourier Transform Spectrum of Cyanamide Using a Multimolecule Ritz Program

The "Ritz" program, originally written for the analysis of the Fourier transform spectra of the methanol isotopomers and presented in previous papers, has been extended in order to analyze the spectra of other molecules. This program evaluates the term values involved in the assigned transitions by the Rydberg-Ritz combination principle, and can tackle such perturbations as Fermi-type resonances or Coriolis interactions. As a first application of the extended version, we present an investigation of the Fourier transform spectrum of cyanamide between 25 and 980 cm-1. More than 16 000 lines have been assigned. Our Ritz database now comprises a list of more than 19 000 assigned lines (including of the microwave and FIR lines available in the JPL database) and more than 3900 term values. All of the lines presented in this paper correspond to transitions within the ground and first excited inversion levels of the ground vibrational state of the small-amplitude modes. Copyright 1998 Academic Press.

The Far-Infrared Spectrum of ClNO2 Studied by High-Resolution Fourier-Transform Spectroscopy

The far-infrared spectrum of nitryl chloride was studied using high-resolution Fourier-transform spectroscopy in the 300-525 cm-1 region. Vibrational band centers of fundamental, hot, and difference bands were determined. Furthermore, rotational and centrifugal distortion constants up to fourth order for the nu3 bands of 35ClNO2 and 37ClNO2 (centered at 370 and 364 cm-1, respectively) were obtained. The nu5 fundamental of ClNO2 (predicted around 410 cm-1) is very weak and overlapped by difference bands. Copyright 1998 Academic Press.