W. Leo Meerts

High resolution UV spectroscopy of phenol and the hydrogen bonded phenol‐water cluster

The S1←S0 000 transitions of phenol and the hydrogen bonded phenol(H2O)1 cluster have been studied by high resolution fluorescence excitation spectroscopy. All lines in the monomer spectrum are split by 56±4 MHz due to the internal rotation of the −OH group about the a axis. The barrier for this internal motion is determined in the ground and excited states; V2″=1215 cm−1, and V2′=4710 cm−1. The rotational constants for the monomer in the ground state are in agreement with those reported in microwave studies. The excited state rotational constants were found to be A′=5313.7 MHz, B′=2620.5 MHz, and C′=1756.08 MHz. The region of the redshifted 000 transition of phenol(H2O)1 shows two distinct bands which are 0.85 cm−1 apart. Their splitting arises from a torsional motion which interchanges the two equivalent H atoms in the H2O moiety of the cluster. This assignment was confirmed by spin statistical considerations. Both bands could be fit to rigid rotor Hamiltonians. Due to the interaction between the overal...

Rotational spectrum and structure of KCN

The spectrum of gaseous KCN was measured in the frequency range between 2 and 39 GHz by microwave absorption and by molecular‐beam electric‐resonance spectroscopy. Combination of the new results with earlier microwave data of KCN in the 100 GHz range made it possible to assign 64 transitions to the ground vibrational state and to fit them to the asymmetric rotor model. The three rotational constants, the five quartic distortion constants, and two sextic distortion coefficients could be determined. Assuming a CN distance of 1.162(10) A we find rKC=2.6(1) A and uKCN=76 °(10). The molecules thus have a nonlinear, T‐shaped structure. The inertial defect gives an estimated value of the lowest vibrational frequency of KCN ω2=157 cm−1, which is in reasonable agreement with ω2=139 cm−1 from matrix‐isolation studies.

Rotationally resolved ultraviolet spectroscopy of indole, indazole, and benzimidazole: Inertial axis reorientation in the S1(1Lb)←S0 transitions

Rotationally resolved laser induced fluorescence excitation spectra of the S1(1Lb)←S0 origin bands of indole, indazole, and benzimidazole have been measured. From these spectra, the rotational constants in both electronic states have been determined. The spectra of all three molecules exhibit ‘‘anomalous’’ rotational line intensities. These intensity perturbations are a result of the reorientation, upon electronic excitation, of the inertial axes of the molecule. Intensity analysis of the rotational lines yielded information about the inertial axis reorientation, and the direction of the transition moment vector for each molecule.

Spectrum of the molecular eigenstates of pyrazine

The molecular eigenstate spectrum belonging to the P and R branches of the1B3u(0-0) electronic transition of pyrazinc-h4 was recorded with a 200 kHz wide laser in a supersome nozzle at a temperature of ≈ 1 K. The square of the former transform fo the amplitude spectrum yields the quantum beats in the fluorescence decay that have been reported before.

The submillimeter rotation tunneling spectrum of the water dimer

Rotational tunneling transitions have been measured for the (Ka = 0, lower) → (Ka = 1, upper) and the (Ka = 1, lower) → (Ka = 2, upper) bands of (H2O)2. Although some of these transitions have been reported in an earlier publication, a more detailed discussion of the experiment and of the results is presented here. Transitions have been measured by direct absorption spectroscopy in a continuous slit nozzle expansion using either harmonics from klystrons or sidebands. These data along with previous measurements have been analyzed using an IAM-like treatment. A better determination of the A rotational constant and of the value of the largest tunneling splitting has been achieved.

Near-uv spectra with fully resolved rotational structure of naphthalene and perdeuterated naphthalene

Abstract The combination of a single-frequency, tunable uv source with a well-collimated supersonic molecular beam and sensitive fluorescence detection has been used to obtain spectra with rovibronic resolution for some large organic molecules. The results of analysis of the 000 and 8 01 vibronic bands of the 1 B 3u ← 1 A g electronic transition for naphthalene and naphthalene-dg are presented. The obtained spectra are assigned using a rigid asymmetric rotor Hamiltonian and the structure in both the ground and electronically excited states is determined. The rotational temperature of the molecules cooled in the beam has been determined. The influence of the nuclear spin statistics on the line intensities is observed and discussed.

Direct determination of molecular constants from rovibronic spectra with genetic algorithms

It is shown that a new procedure, based on genetic algorithms (GA’s), can be used for direct determination of molecular constants, in particular rotational constants, from rovibronic spectra. This new approach only requires an estimate of the acceptable range of the parameters. The power of the method is demonstrated on the rotationally resolved fluorescence spectra of indole, indazole, benzimidazole, and 4-aminobenzonitril. A rigid asymmetric rotor Hamiltonian is used to calculate the theoretical spectra. The GA matches the generated spectra with an experimental spectrum with the use of a new method for spectra comparison. This spectra comparison function is able to deal with frequency shifts which are caused by (small) changes in the rotational constants and it yields better results in comparison with traditional spectra comparison methods, like RMS. In addition, the robustness of the method is tested.

High-resolution molecular-beam spectroscopy of NaCN and Na13CN

Abstract The sodium cyanide molecule was studied by molecular-beam electric-resonance spectroscopy in the microwave region. We used the seeded-beam technique to produce a supersonic beam with strong translational, rotational and vibrational cooling. In the frequency range 9.5–40 GHz we observed and identified for NaCN 186 and for Na13CN 107 hyperfine transitions in 20 and 16 rotational transitions, respectively, all in the ground vibrational state. The rotational, the five quartic and three sextic centrifugal distortion constants of NaCN are: A″ = 57921.954(7) MHz; B″ = 8369.312(2) MHz, C″ = 7272.712(2) MHz. All quadrupole and several spin-rotation coupling constants for the hyperfine interaction were evaluated. The quadrupole coupling constants (in MHz) for NaCN are: eQq12(Na) = −5.344(5), eQq12 = 2.397(7). eQq12(N) = 2.148(4), eQq12(N) = −4.142(5). From these constants and those of Na13CN we have determined the principal components of the quadrupole coupling tensor for potassium and nitrogen. The structure of sodium cyanide evaluated from the rotational constants of NaCN and Na13CN was found to be T shaped, similar to the structure of KCN but completely different from the linear isocyanide configuration of LiNC. The effective structural parameters for sodium cyanide in the ground vibrational state are: rCN = 1.170(4) A, rNaC = 2.379(15) A, rN12N = 2.233(15) A, in gratifying agreement with ab initio calculations. Both the geometrical structure and the hyperfine coupling justify the conclusion that the CN group in gaseous sodium cyanide approximately can be considered as a free CN− ion.

Vibronic coupling in indole: I. Theoretical description of the ¹La-¹Lb interaction and the electronic spectrum

The properties of the three lowest singlet electronic states (ground, (1)L(b), and (1)L(a) states) of indole (C(8)H(7)N) have been calculated with second-order approximate coupled-cluster theory (CC2) within the resolution-of-the-identity approximation. Refined electronic energies at the CC2 optimized structures and transition dipole moments were calculated using a density functional theory multi-reference configuration-interaction (DFT/MRCI) approach. Structures, energies, and dipole moments are reported for all three states and compared to experimental values. From the optimized structures and calculated transition dipole moments, we predict that pure (1)L(b) bands will have positive signs for both the axis reorientation angle theta(T) and the angle theta of the transition dipole moment with respect to the inertial a axis. For (1)L(a) bands the signs of both angles will be reversed. Vibronically coupled bands can exhibit opposite signs for theta and theta(T). The absorption and emission spectra of indole are calculated based on the Franck-Condon Herzberg-Teller approximation using numerical transition dipole moment derivatives at the DFT/MRCI level of theory. Implications for the experimentally observed vibronic spectra are discussed. Predictions are made for rotationally resolved spectra of various rovibronic bands. A conical intersection, connecting the (1)L(b) and (1)L(a) states, which can be accessed to varying extents via different Herzberg-Teller active modes is found approximately 2000 cm(-1) above the (1)L(b) minimum.

Ultra high‐resolution fluorescence excitation spectrum of 1B1 pyrimidine in a molecular beam. Structural assignments, analysis of singlet–triplet perturbations, and implications for intersystem crossing in the isolated molecule

We have observed, and assigned, the fluorescence excitation spectrum of the 000 band in the 1B1←1A1 electronic transition of pyrimidine, at a resolution of ∼10 MHz. The rotational constants of the 1B1 state, the lowest excited singlet state, are A’=6352±3, B’=5853±3, and C’=3042.0±0.5 MHz. The magnitudes of these constants are not very different from those of the ground (1A1) state. However, the in‐plane a and b inertial axes in the 1B1 state are rotated by 90° with respect to those of the 1A1 state. The spectrum also exhibits numerous perturbations, evidenced by the presence of extra lines, anomalous intensities and lifetimes, and shifts of the main lines from their expected positions. The perturbations are strongly magnetic‐field dependent, demonstrating that they arise from an intramolecular coupling of the 1B1 state with nearly isoenergetic rovibronic levels of a lower triplet (3B1) state. Models are proposed to account for this behavior based on a deconvolution of the experimental spectrum and simula...