Yuan‐Pern Lee

Diatomic sulfur: Low lying bound molecular electronic states of S2

We present here the results of self‐consistent field (SCF) and configuration interaction (CI) type calculations on thirteen low‐lying electronic states of diatomic sulfur. The basis set was one of double zeta quality augmented with polarization functions. The CI space for each electronic state consisted of all configurations constructed from single and double excitations of electrons from the valence orbitals of the Hartree–Fock configuration. There are several significant findings of this study. First, we report the discovery of a previously unobserved and bound 1Πu state which lies approximately 37u2009000 cm−1 above the ground state. This state dissociates to two ground state sulfur atoms. Second, we provide new predictions of excitation energies and properties for the three states e1Πg, c1Σ−u, and B′3Πu. These states were suspected or known to be bound, but experimentally determined properties were uncertain. Finally, we find that systematic application of a formula of Davidson, which estimates the contri...

Sulfur oxide: Low‐lying bound molecular electronic states of SO

We present here the results of self‐consistent field (SCF) and configuration interaction (CI) theoretical studies of seven low‐lying electronic states of sulfur oxide. The basis set was of double zeta quality augmented with polarization functions. The CI space for each electronic state consisted of all configurations constructed from single and double substitutions of electrons from the valence orbitals of the Hartree–Fock reference occupation. Spectroscopic constants as well as dipole moments for each electronic state were predicted both at the SCF and CI levels of theory. Of particular significance is the prediction of excitation energies and properties for three low‐lying states for which experimental information is either unavailable or only very recently available. These states are the c1Σ− state (Te=28u2009100±300 cm−1), the A′ 3Δ state (29u2009200±300 cm−1) and the A″ 3Σ+ state (30u2009200±300 cm−1).

Chemiluminescence of SO (? 1Σ −→ ? 1Δ) in solid argon

Matrix samples of OCS/O2/Ar (1/1/200), OCS/NO2/Ar (1/1/200), and SO2/Ar (1/100) were photolyzed at 10u2009°K. Subsequent slow warming produced previously observed thermoluminescence due to S2 and SO2. In addition, a new progression was observed between 5200 and 7700 A, and, for OCS/O2/Ar samples, a broad emission peak at 7816 A. With 18O2, the bands in the progression displayed isotopic shifts that permit deduction of ν00=21u2009363 cm−1, ωe′′=1097 cm−1, and ωexe′′=7 cm−1. By analogy to the energy levels of O2 and S2, it is possible to assign the progression to the ?u20091Σ−→?u20091Δ transition of sulfur monoxide SO. This is the first observation of this transition of SO, and it provides the first spectroscopic information about the ?u20091Σ− state. The 7816 A peak is attributed to emission by sulfur atom 1S→1D. The excitation of sulfur atoms may occur through energy transfer; if so, some of this transfer is from the chemically excited SO2* (?u20093B1) molecule.

Chemiluminescence of ethylene in an inert matrix and the probable infrared spectrum of methylene

Photolysis of diazomethane in solid matrices at 6 or 10u2009°K and its subsequent thermoluminescence have been reinvestigated. At 11–12u2009°K, the thermoluminescence consists of a broad emission at 5220±4 A and a weak progression with spacing 338 cm−1 in the 6000–7600 A region, the latter probably due to an impurity. Above 15u2009°K, the spectrum is dominated by a broad emission (180 cm−1 FWHM) centered at 5987 A, the red glow detected earlier. The emitter responsible for the 5220 A feature is not clear, though the frequency is close to that of the atomic N(2D→4S) transition. The 5987 A emission is best assigned to the C2H4(Z or V→T) transition excited through the reaction of CH2(3B1) molecules either with CH2 or CH2NN. This C2H4 transition has not previously been accessible to experiment. The infrared spectra of the photolysis products at 10u2009°K indicate that absorptions at 1115 cm−1 (Ar) and 1109 cm−1 (Xe) can be attributed to ν2, the bending mode of CH2 in its ground state. This assignment may be of significance i...

Formic acid chemiluminescence from cryogenic reaction between triplet methylene and oxygen

Matrix samples of CH2N2/O2/Ar were photolyzed at 8u2009°K. Subsequent slow warming produced the previously observed O2 u2009A′u20093Δu→Xu20093Σg− thermoluminescence and two new progressions, one at 3100–3600 A and one at 3900–4900 A. With 18O2, the 3100–3600 A progression displayed isotopic shifts that allow two possible v″ vibrational assignments to give ν00 = 34u2009730 or 33u2009340 (±40) cm−1 and ωe″ = 1390±15 cm−1. Similarly, the 18O2 shifts in the 3900–4900 A progression lead to two possible v″ assignments to give ν00 = 26u2009230 or 25u2009100 (±60) cm−1 and ωe″ = 1124±40 cm−1. These spectroscopic parameters point to assignments of the a3A″→Xu20091A′ and A′u20091A′→Au20091A″ transitions of HCOOH for the 3100–3600 and 3900–4900 A progressions, respectively. These are the first observations of such transitions of formic acid and are attributed to the CH2(3B1)+O2(3Σg−) reaction in the cryogenic matrix.Matrix samples of CH2N2/O2/Ar were photolyzed at 8u2009°K. Subsequent slow warming produced the previously observed O2 u2009A′u20093Δu→Xu20093Σg− thermoluminescence and two new progressions, one at 3100–3600 A and one at 3900–4900 A. With 18O2, the 3100–3600 A progression displayed isotopic shifts that allow two possible v″ vibrational assignments to give ν00 = 34u2009730 or 33u2009340 (±40) cm−1 and ωe″ = 1390±15 cm−1. Similarly, the 18O2 shifts in the 3900–4900 A progression lead to two possible v″ assignments to give ν00 = 26u2009230 or 25u2009100 (±60) cm−1 and ωe″ = 1124±40 cm−1. These spectroscopic parameters point to assignments of the a3A″→Xu20091A′ and A′u20091A′→Au20091A″ transitions of HCOOH for the 3100–3600 and 3900–4900 A progressions, respectively. These are the first observations of such transitions of formic acid and are attributed to the CH2(3B1)+O2(3Σg−) reaction in the cryogenic matrix.

Chemiluminescence of S2 in solid argon

Upon slow warming of previously photolyzed matrix samples of OCS/Ar, CS2/Ar, OCS/Kr, and OCS/SF6 (M/R=100), the B (B″) →X chemiluminescence of S2 is observed as reported earlier. In addition, two new progressions were recorded, a weak progression between 5580–6890 A and a stronger one between 6680–8750 A. With OC34S, the 6680–8750 A progression displayed isotopic shifts that allow deduction of νoo=15u2009750±10 cm−1, ω″e=699±5 cm−1, and ωexe″=2.6±1 cm−1. Similarly, the weaker 5580–6890 A progression corresponds to νoo=20u2009870±30 cm−1, ω″e=724±6 cm−1, and ωex″e=2.9± 1 cm−1, with a less favored, alternative vibrational assignment that gives νoo=21u2009600±30 cm−1, ω″e=731±6 cm−1 and ωex″e=2.9±1 cm−1. These spectroscopic parameters point to assignments of the c1Σ−u→a1Δg and A′3Δu →X3Σ−g transitions of S2 for the 6680–8750 A and 5580–6890 A emissions, respectively. These are the first observations of these transitions of diatomic sulfur, and one of them provides the first direct spectroscopic information illustrating ...

The chemiluminescent reactions Ba+N2O and Ba+O3 in solid argon

Visible chemiluminescence in the spectral region 5500–7300 A due to the Ba+N2O and Ba+O3 reactions is observed in argon matrices at temperatures below 15 K. Emission is recorded both as thermoluminescence and during deposition. Two molecular progressions of BaO, with ν00=17u2009035 and 17u2009195u2009cm−1, and ω″e=657u2009andu2009651u2009cm−1, are found and are tentatively attributed to emission in the au20093Σ+→Xu20091Σ+ transition for BaO* in different matrix sites, though the Au20091Σ+→Xu20091Σ+ transition could also be involved. The presumed excitation mechanism is the reactive population of one or both of the singlet states Au20091Σ+ and/or A′u200a1Π followed by rapid intersystem crossing to the au20093Σ+ state. This is the first observation of a chemiluminescent atom transfer reaction in low temperature matrices. Emission due to Ba atoms in the (6s6p1P) →(6s2u20091S) transition is also observed, apparently excited by energy transfer from BaO*, most probably from the A′u20091Π state.

Three‐dimensional variable‐angle nuclear magnetic resonance exchange spectroscopy without rotor axis hopping

Slow, large-amplitude chain motions play an important role in determining the macroscopic mechanical properties of polymers. Although such motions have been studied quantitatively by two-dimensional (2D) nuclear: magnetic resonance (NMR) exchange experiments, overlapping anisotropic patterns hamper spectral analysis, and limit applications. Variable angle correlation spectroscopy (VACSY) has proven useful in resolving such problems for rapidly spinning samples by separating anisotropic spectral patterns according to isotropic chemical shifts. In a previous study [J. Am. Chem. Sec. 115, 4825 (1993)], we described a three-dimensional (3D) NMR experiment that incorporates; the VACSY method and a hop of the rotor axis to correlate the isotropic chemical shifts to 2D anisotropic exchange patterns. The hop of the rotor axis, however, presents experimental difficulties and limits the range of motional rates that may be studied. We present in this paper a new 3D VACSY exchange experiment that obtains the same correlations without the need for the rotor axis hop. A series of 2D exchange spectra are recorded with the sample spinning at different rotation axis angles. Then using the scaling of the anisotropic frequency at the different angles, we construct the data onto a 3D matrix so that a Fourier transformation directly yields the desired correlations. The technique is applied to C-13 exchange NMR to study the slow molecular motion of ordered isotactic polypropylene.

Carbon-13 chemical shift tensor correlation via spin diffusion in solid tropolone using switched-angle spinning spectroscopy

In switched‐angle spinning spectroscopy (SAS) a sample is spun about different angles, β, relative to the magnetic field, during various periods of the experiment. In the present work, SAS is combined with two‐dimensional exchange spectroscopy in order to correlate carbon‐13 chemical shift tensors of the carbonyl (1) and hydroxyl (2) carbons of tropolone. Experiments were performed on a sample enriched to 25 at.u2009% in each of these sites (at different molecules). At this level of enrichment the dominant exchange mechanism between the two sites involves spin diffusion. The experiment consists of a preparation period during which the sample spins at the magic angle and the magnetization of one of the sites is quenched by means of a selective pulse sequence. During the rest of the experiment the sample spins with its axis away from the magic angle except for a short period just before the detection where the axis is switched to the magic angle in order to select the magnetization to be detected. Experiments w...