Vacuum ultraviolet absorption spectra of aromatic molecules in solution

The absorption spectra of benzene, naphthalene, azulene, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), pyrene, and fluoranthene were measured in liquid perfluoro-n-hexane (PFn-H) in the range 35 000–65 000 cm^?1 and in liquid n-pentane (n-Pt) in the range 35 000–59 000 cm^?1, using a 150-μm-thick cell. The absorption spectra in PFn-H show more structure than those in n-Pt and because they are extended in the Rydberg transition region may be used to separate Rydberg from core transitions in the gaseous spectra of the molecules studied. The transmission spectra for PFn-H, n-Pt, and some of the most commonly used hydrocarbon solvents are presented also.     

Rydberg series of NO converging to the ionic states b3Π, A1Π, and w3Δ

Rydberg series of NO in the 600–1000-Åregion were investigated by using a 6.65-m high-dispersion vacuum spectrograph. The previous β, γ (v = 0), and γ (v = 1) Rydberg series were extended up to n = 31, 31, and 28, respectively. From the analysis of these Rydberg series, accurate ionization energies were obtained: 133 565 ± 3 (16.5596 eV) for b³Π (v = 0); 147 811 ± 3 (18.3258 eV) for A¹Π (v = 0); 149 372 ± 3 cm^?1 (18.5193 eV) for A¹Π (v = 1) of NO⁺. A new Rydberg series converging to one of the triplet components in b³Π was identified, and the coupling constant of b³Π was estimated to be 35 ± 8 cm^?1. Higher members of two Rydberg series were newly observed in the 700–725-Åregion. From their series limits, 139 926 ± 3 and 141 160 ± 3 cm^?1, they were assigned to be the Rydberg series converging to the v = 3 and 4 levels of ω ³Δ in NO⁺.     

Photo-absorption studies on carbonyl sulphide in 30,000–91,000 cm−1 region using synchrotron radiation

Photo-absorption spectrum of carbonyl sulphide (OCS) is recorded in 30,000–91,000 cm^?1 (3300–1050 Å) region at an average resolution of 1.2 Å using Photo-physics beamline on the 450 MeV Indus-1 synchrotron radiation source at RRCAT Indore, India. Owing to significant absorption cross section dependence, spectra of OCS are recorded at various pressures (0.001–5 mbar) to optimize the S/N ratio for band systems appearing at different energy regions. The spectral region below 70,000 cm^?1 has contributions from dissociation mechanism of the ground state of OCS and three valence band systems arising from promotion of a 3π electron to 4π and 10σ orbital. Improved S/N ratio helped in unambiguous assignment of the valence band progressions at 42,000–48,000 cm^?1, 53,000–62,000 cm^?1 and 63,500–70,000 cm^?1 regions to the ¹Δ←X¹Σ⁺ transition, the relatively intense and sharp bands of ¹Π←X¹Σ⁺ transition and intense but broad bands of ¹Σ⁺←X¹Σ⁺ transition, respectively, and obtain the vibrational frequencies. Above 70,000 cm^?1 Rydberg series arising from s, p, d and f orbitals converging to the ionic ground state X²Π of OCS⁺ (90,121 cm^?1) are identified. Long progression in the first few members of the Rydberg series is suggestive of mixed valence character. Quantum defects are evaluated and used to discuss the nature of the molecular orbital. The present study provides a unifying picture of the VUV photo-absorption spectrum of OCS up to its first ionization limit.     

Magnetic Circular Dichroism Spectra of p-Disubstituted Benzenes Possessing Both Electron-Donating and Accepting Groups

Abstract

The magnetic circular dichroism (MCD) and absorption spectra of some p-disubstituted benzenes possessing both electron-donating and accepting groups were measured. The electronic spectra of the above compounds are characterized by the appearance of two absorption bands, one of which appears at 25000–40000 cm^?1 region with large intensity, and another at 40000–50000 cm^?1 region with rather small intensity. These absorption bands, especially the latter one, have been investigated by means of the MCD spectra and molecular orbital calculations based on the Pariser-Parr-Pople method.     

Higher electronic states of ReF6

The ultraviolet absorption of ReF₆ comprises a broad (δ⁻ = 3760 cm⁻¹) structureless band centered at 47 687 cm⁻¹ and two structured systems originating at 49 322 and 56 404 cm⁻¹. The first is interpreted as a charge transfer transition. The other two are interpreted as members of a Rydberg series converging on an ionization potential of 7.99 eV. The molecule, in the Rydberg states, is a slightly expanded octahedron     

The structure of the triethylenediamine molecule in an excited electronic state

Ultraviolet absorption spectrum has been observed of triethylenediamine (1,4 diazabicyclo[2.2.2]octane) ($\text{D}{}_{\mathrm{<mtext>3</mtext><mtext>A</mtext><mtext>?</mtext>}}$) vapor in the 2200–2600 Å region, and an analysis has been made of its vibrational structure. The vibronic 0-0 transition was determined to be at 2513.65 Å (39782.8 cm^?1), and in the 2540–2590 Å region there were four hot bands found: two from the a₁′-type vibrational levels and two from the a₂″-type vibrational levels of the ground electronic state. The fine structures of these hot bands were examined with 0.49 Å/mm dispersion (slit width = 50 μm). For each of the two a₂″-type hot bands, a progression with 30 ～ 40 cm^?1 spacings was observed; whereas, no such progressions were found for the a₁′-type hot bands, in which absorption peaks are much more concentrated within narrow ranges. These progressions were attributed to a large-amplitude twisting motion of triethylene-diamine molecule. For explaining the whole energy-level structure, another large-amplitude motion, has been postulated. That is a double-minimum deformation motion along the molecular axis; in each minimum, one of the two NC₃ umbrellas is half-way open and the other NC₃ umbrella is half-way closed. The height of the potential barrier between these two minima has been estimated to be 1586 cm^?1.     

The electronic states of silicon monofluoride: Rydberg states

SCF-CI calculations are reported for the Rydberg states converging to the ground state of the SiF⁺ ion. The adjustment between the observed and calculated values for the energy of the first Rydberg B²Σ⁺ state has allowed the characterization of 15 observed ²Σ⁺ and ²Π Rydberg states. A value of 58960 cm^?1 has been obtained for the ionization potential of SiF. The A²Σ⁺ state appears to be a valence state with some Rydberg character.     

High-resolution optogalvanic study of the c4(0)1Πu, c′5(0)1Σu+, and a″(0)1Σg+ Rydberg states of N2

A new optogalvanic technique with an rf discharge was applied to a high-resolution study of the Rydberg states of N₂. The Ledbetter band, c₄(0)¹Π_u ← a″(0)¹Σ_g⁺, and a new visible band, c′₅(0)¹Σ_u⁺ ← a″(0)¹Σ_g⁺, were studied at a Doppler-limited resolution of 0.05 cm^?1. A Doppler-free method was also applied to resolve overlapped lines. Precise wavenumbers were determined for the rotational transitions of the two Rydberg bands. The rotational and the centrifugal constants for the lowest Rydberg state, a″(0)¹Σ_g⁺, were determined to be B₀ = 1.913748(42) cm^?1 and D₀ = 6.088(99) × 10^?6 cm^?1, where the numbers in parentheses are the standard deviation and apply to the last digits.     

The polarized reflection and absorption spectra of perylene crystals in monomeric and dimeric forms

The electronic absorption spectra of perylene crystals in the α- and β-forms were measured by the normal incidence reflection method in the spectral region from 20 000 to 60 000 cm^?1. From the absorption spectrum polarized perpendicular to [1$\text{1}$0] axis of the α-form crystal, the bands around 24 000 cm^?1 were determined to be polarized along the long molecular axis. Two strong bands with different polarizations were observed around 50 000 cm^?1 for each of the α- and β-perylene crystals and were assigned to the transitions to the ¹B_2u and ¹B_3u states. The observed polarized absorption spectra as a whole were consistent with the theoretical results by Hummel and Ruedenberg and the reflection method was found to be suitable to the polarized absorption measurement of strong bands of crystals. The observed factor-group splittings were compared with the theoretical values, the oriented gas model being found to be applicable to the β-form crystal.     

The high resolution infrared spectrum of the 2ν2 + ν3 and ν1 + ν2 + ν3 bands of 14N16O2: Vibration and vibration-rotation constants of the electronic ground state of 14N16O2

The A-type bands, 2ν₂ + ν₃ and ν₁ + ν₂ + ν₃, with band centers at 3092 and 3638 cm^?1, respectively, of ¹⁴N¹⁶O₂ have been measured with resolution of 0.03 cm^?1 or better. Spectroscopic constants have been derived for the upper states of both bands. Infrared determined band constants have been combined with laser-excited resonance fluorescence data to obtain a set of vibration and vibration-rotation constants for the ground state of ¹⁴N¹⁶O₂.     

Doppler-limited spectra of the CH stretching fundamentals of formaldehyde

The Doppler-limited spectrum of H₂CO in the 2700 to 3000 cm^?1 region has been recorded using a tunable difference-frequency laser system. This region encompasses the ν₁ and ν₅ fundamental CH stretching bands of formaldehyde as well as a number of strongly interacting combination bands. The tunable laser spectrometer affords complete spectral coverage with a calibration precision better than 10^?3 cm^?1 for the transition frequencies and with absolute absorption intensity measurements better than 2%. The band centers for the ν₁ and ν₅ vibrations are measured to be 2782.4572 ± 0.0010 cm^?1 and 2843.3254 ± 0.0015 cm^?1 respectively, independent of the upper-state rotational constants.     

Wavenumbers,line strengths,and assignments in the Doppler-limited spectrum of formaldehyde from 2700 to 3000 cm−1

The spectrum of H₂CO from 2700 to 3000 cm^?1 has been examined at Doppler-limited resolution using a tunable difference frequency laser spectrometer at Lincoln Laboratory. The wavenumbers and strengths of 4350 absorptions have been determined with an accuracy of 0.001 cm^?1 and 5%, respectively. These data have been incorporated into the analysis of lower-resolution data from Florida State University to assign 72% of the observed absorptions to one of seven bands: ν₃ +ν₄ (a C-type band at 2655 cm^?1), ν₃ + ν₆ (a B-type band at 2719.156 cm^?1), ν₁ (an A-type band at 2782.457 cm^?1), ν₅ (a B-type band at 2843.326 cm^?1), ν₂ + ν₄ (a C-type band at 2905 cm^?1), 2ν₃ (an A-type band at 2999.5 cm^?1) and ν₂ + ν₆ (a B-type band at 3000.066 cm^?1). The band ν₃ + ν₄ has been observed for the first time, and the band center for 2ν₃ has been corrected from a value of 2972 cm^?1 to the value listed above. The effects of strong Fermi and Coriolis resonances on the spectra are discussed.     

Observations on the electronic spectrum of gaseous FeO

About 400 lines are assigned to FeO emission bands in the region 5580 to 6265 Å. The lower state of all the bands analyzed is identified as the ground state of the molecule, for the value of the lower-state vibration frequency (ω_e = 880.61 ± 0.02 cm^?1) is in excellent agreement with that observed in low-temperature matrix isolation, itself confirmed by isotopic substitution. This state is also observed as the lower state in laser-induced fluorescence. However, 880 cm^?1 is significantly smaller than the value found in laser photodetachment studies of FeO^? (970 ± 60 cm^?1). The rotational analysis is consistent with a parallel transition, ΔΛ = 0, but the value of Λ is not determined. According to theoretical calculations, the orange bands most probably arise from a ⁵Σ-⁵Σ transition. There is at least one nearby excited state, for all analyzed upper levels are perturbed.     

Vacuum ultraviolet absorption spectra of dichlorosilane,dichloromethylsilane and dichlorodimethylsilane

The vapor phase vacuum ultraviolet absorption spectra of dichlorosilane, dichloromethylsilane and dichlorodimethylsilane are reported for the region from 40 000 to 83 000 cm^?1 (250-120 nm). Absorptions of these compounds are assigned as primarily Rydberg excitations of chlorine valence non-bonded electrons. The σ* ← np(SiCl) transition for these compounds is the first observed absorption and is badly overlapped with the first Rydberg absorptions. The first three ionization potentials of dichloromethylsilane were calculated to be 11.47, 11.82, and 12.28 eV using averaged term values and transition energies. The analysis of these spectra revealed that the first p and d molecular Rydberg states appear to be nearly degenerate and that these Rydberg levels are greatly destabilized with methyl substitution. The results of this study support the presence of strong d-p(SiCl) interactions. The strength of this effect is compared in dichlorosilane and dichlorodimethylsilane using an empirical relation.     

Carbon suboxide: The infrared spectrum from 1800 to 2600 cm−1

The infrared spectrum of carbon suboxide has been recorded from 1800 to 2600 cm^?1 at a resolution of 0.003 cm^?1. About 7% of the ca. 40 000 lines observed have been assigned and analyzed, belonging to 36 different bands. Most of these are associated with the fundamental ν₃, at 2289.80 cm^?1, and the combination band ν₂ + ν₄, at 2386.61 cm^?1, each of which give rise to a system of sum bands, difference bands, and hot bands involving the low-wave-number fundamental ν₇ at 18 cm^?1. A few other tentative assignments are made. The bands have been analyzed for vibrational and rotational constants.     

Vibration-rotation spectrum of N2O in the region of the lowest fundamental ν2

The high resolution infrared spectrum of N₂O in the region of ν₂ has been studied with a Fourier transform spectrometer at a resolution of (about) 0.005 cm^?1 and an accuracy of about ±0.00005 cm^?1. In addition to ν₂, “hot” bands associated with this band and the bending fundamental ν₂ of ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁵N¹⁶O were analyzed.     

Fourier transform spectroscopy of new emission systems of NbN in the visible region

The emission spectrum of NbN has been reinvestigated in the 8000-35 000  cm⁻¹ region using a Fourier transform spectrometer and two groups of new bands were observed. The bands observed in the 18 000-20 000 cm⁻¹ region have been assigned to a new ³Π-X³Δ transition. Three bands with R heads near 19 463.8, 19 659.0 and 19 757.0 cm⁻¹ have been assigned as 0-0 bands of the ³Π₂-X³Δ₃, ³Π₁-X³Δ₂ and ³Π_0±-X³Δ₁ subbands, respectively, of this new transition. Three additional ΔΩ = 0 bands have been observed in the 24 000-26 000  cm⁻¹ region. A 0-0 band with an R head near 25 409.9 cm⁻¹ has been assigned as a ΔΩ = 0 transition having X³Δ₂ as its lower state while two additional bands with heads near 25 518.7 and 25 534.8 cm⁻¹ were found to be ΔΩ = 0 bands having X³Δ₁ as the common lower state. Two of these three bands are perhaps subbands of a ³Δ-X³Δ transition. Most of the excited levels are affected by perturbations.     

The Rydberg spectroscopy of methyl chloride

The absorption spectrum of methyl chloride has been studied in the energy region from 68 600 to 94 760 cm^?1. With the exception of a valence-shell transition, all transitions were interpreted as molecular Rydberg transitions. These assignments were made utilizing the Rydberg formula. The similarity of the vibrational fine structures observed in the photo-electron spectrum of methyl chloride and its VUV spectroscopy is shown to further support the Rydberg assignments.     

Vibrational Spectroscopic and Theoretical Studies of Urea Derivatives with Biochemical Interest: N,N′-Dimethylurea,N,N,N′,N′-Tetramethylurea,and N,N′-Dimethylpropyleneurea

Abstract

Mid-infrared, far-infrared, and Raman vibrational spectroscopic studies were combined with density functional theory (DFT) calculations and normal coordinate force field analyses for N,N′-dimethylurea (DMU), N,N,N′,N′-tetramethylurea (TMU), and N,N′-dimethylpropyleneurea (DMPU: IUPAC name 1,3-dimethyltetrahydropyrimidin-2(1H)-one). The equilibrium molecular geometry of DMU (all three conformers), TMU, and DMPU and the frequencies, intensities, and depolarization ratios of their fundamental infrared (IR) and Raman vibrational transitions were obtained by DFT calculations. The vibrational spectra were fully analyzed by normal coordinate methods as well. A starting force field for DMPU was obtained by adapting corresponding force constants for DMU and TMU, resulting after refinements in the stretching force constants C=O (7.69, 7.30, 7.68 N·cm^?1), C–N (5.16, 5.55, 5.05 N·cm^?1), and C-Me (5.93, 4.00, 4.22 N·cm^?1) for DMU, TMU, and DMPU, respectively. The dominating conformer of liquid DMU was identified as trans-trans, strong intermolecular hydrogen bonding was verified in solid DMU, and weak dipole–dipole association was found in liquid TMU and in DMPU. Special attention was paid to analyzing the methyl group frequencies, which revealed deviations from local C_3v symmetry. A linear correlation was found between the CH stretching force constants and the inverse of the CH bond lengths (1/r ²). The averaged NH stretching frequencies of gaseous, dissolved, and solid urea and of DMU, with variations for hydrogen bonding of different strength, are linearly correlated to the NH stretching force constants. Characteristic skeletal vibrations were assigned for a broad variety of urea derivatives and also for pyrimidine derivatives, which all contain the N₂C=O entity. The very strong IR bands of C=O stretching (1,676 ± 40 cm^?1) and asymmetric CN₂ stretching (1,478 ± 60 cm^?1), and the very intense Raman feature of symmetric CN₂ stretching or ring breathing (757 ± 80 cm^?1), can be recognized as fingerprint bands also for the pyrimidine derivatives cytosine, thymine, and uracil, which all are nucleobases in DNA and RNA nucleotides.