Photooxidation Reactions of Small-Chain Methyl Esters, Aerosol Photoelectron Spectroscopy, and the Photodissociation of Ethylenediamine

Research conducted at the University of San Francisco is presented within this masters thesis, including the Cl-initiated photooxidation reactions of methyl propanoate, methyl butanoate, and methyl valerate (pentanoate), the aerosol photoelectron spectroscopy of isoprene and gamma-valerolactone, and the TPEPICO investigation of ethylenediamine. Experiments were conducted at the Lawrence Berkeley National Lab Advanced Light Source (ALS), the National Synchrotron Radiation Research Center in Hsinchu City (NSRRC), and the University of Stockton in California, respectively

Electronic spectra of positively charged carbon clusters - C2n+ (n=6-14)

Electronic spectra are measured for mass-selected C+2( = 6–14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N2 molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C+2( = 6–14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C+22 and larger clusters. The C+2 electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C+12, C+16, C+20, C+24, and C+28 are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C+14, C+18, C+22, and C+26 are narrower, consistent with slower non-radiative deactivation

Primary care nurses: effects on secondary care referrals for diabetes

Background: Primary care nurses play an important role in diabetes care, and were introduced in GP-practice partly to shift care from hospital to primary care. The aim of this study was to assess whether the referral rate for hospital treatment for diabetes type II (T2DM) patients has changed with the introduction of primary care nurses, and whether these changes were related to the number of diabetes-related contacts in a general practice. Methods: Healthcare utilisation was assessed for a period of 365 days for 301 newly diagnosed and 2124 known T2DM patients in 2004 and 450 and 3226 patients in 2006 from general practices that participated in the Netherlands Information Network of General Practice (LINH). Multilevel logistic and linear regression analyses were used to analyse the effect of the introduction of primary care nurses on referrals to internists, ophthalmologists and cardiologists and diabetes-related contact rate. Separate analyses were conducted for newly diagnosed and known T2DM patients. Results: Referrals to internists for newly diagnosed T2DM patients decreased between 2004 and 2006 (OR:0.44; 95%CI:0.22-0.87) in all practices. For known T2DM patients no overall decrease in referrals to internists was found, but practices with a primary care nurse had a lower trend (OR:0.59). The number of diabetes-related contacts did not differ between practices with and without primary care nurses. Cardiologists’ and ophthalmologists’ referral rate did not change. Conclusions: The introduction of primary care nurses seems to have led to a shift of care from internists to primary care for known diabetes patients, while the diabetes-related contact rate seem to have remained unchanged.

Electronic spectroscopy of small carbocations

© 2020 Giel MullerThis work focuses on the measurement and analysis of gas-phase electronic spectra of C4H2+, C4H4+, C4H5+, and C6H4+ cations. The spectra are recorded by photodissociating the cations, or their messenger tagged complexes, in a tandem mass spectrometer and monitoring the photofragment signal as a function of laser wavelength. Ultimately, structural and energetic information extracted from these spectra, complemented by quantum chemical calculations, may provide insights into the roles of cations in the chemistry of flames, plasmas, and extraterrestrial environments. Understanding the vibronic structures of the cations is also fundamental for facilitating their possible detection in remote environments. Electronic spectra of C4H2+-Ar_n (n = 1-3) and C4H2+-(N2)_n (n = 1-2) complexes are recorded over the 290-530 nm range by monitoring C4H2+ photofragments. Intense narrow bands in the visible region arise from the A2(Pi)u<--X2(Pi)g electronic transition of the diacetylene cation, HC4H+. Vibronic transitions are compared with previous results and theoretical predictions for the A2(Pi)u<--X2(Pi)g band system based on calculations at the CCSD/cc-pCVTZ and EOM-CCSD/cc-pCVTZ levels of theory. Hole burning experiments confirm that the weak, broad bands in the ultraviolet region also correspond to excitations of HC4H+. These bands are assigned to the 22(Pi)u<--X2(Pi)g electronic transition based on calculated energies and oscillator strengths, previous experiments, and spectra of isoelectronic molecules. The origin transition is observed at 29723 cm-1 (336.44 nm) for HC4H+-Ar, followed by a progression in the symmetric C-C stretch vibration, spaced by 906 cm-1. Spectral congestion observed toward shorter wavelengths is possibly due to the presence of close-lying electronic states, vibronic coupling effects, and HC4H+ being bent in the 22(Pi)u electronic state. Electronic spectra of Ar- and N2-tagged C4H4+ cations, generated from acetylene ion-molecule reactions, are recorded over the 296-550 nm range by monitoring C4H4+ and C4H3+/C4H2+ photofragments. The spectra exhibit a clearly resolved band system commencing at 511 nm, a set of weak bands around 410 nm, and an intense broad band in the UV. Band assignments are based on previous spectroscopic studies and calculations for the four lowest energy C4H4+ isomers. The visible region of the spectrum is dominated by the B2A’’<--X2A’’ electronic transition of the vinylacetylene cation (VA+) whose origin transition occurs 19558 cm-1 (511.30 nm) for VA+-Ar and at 19551 cm-1 (511.48 nm) for VA+-N2. The spectrum features a strong progression in the central C-C stretching vibration spaced by approximately 806 cm-1 and well defined vibronic transitions associated with two bending vibrational modes. The weaker 410 nm system is assigned to the D2B3<--X2B2 electronic transition of the butatriene cation (BT+). Two broad bands, following the origin transition at 24399 cm-1 (409.85 nm) in the C4H4+-Ar spectrum, arise from excitation of a symmetric C=C stretching vibration and possibly the CH2 twisting vibration. The observation of C4H4+ and C4H3+ /C4H2+ photofragments following excitation of BT+-Ar complexes over the 380-410 nm range accords with calculated dissociation pathways on the ground state C4H4+ potential energy surface. The broad band appearing at wavelengths below 320 nm is predicted to be due to electronic transitions of all four of the lowest energy C4H4 + isomers, including the VA+ and BT+ cations. The B2A’<--X2A’ electronic spectrum of the 1-butyn-3-yl cation (H3CCHCCH+) is recorded over the 245-285 nm range by photodissociating the bare cation and its Ne and Ar-tagged complexes. Origin transitions for H3CCHCCH+ and H3CCHCCH+-Ne are observed at 35936 cm-1 (278.27 nm) and 35930 cm-1 (278.32 nm), respectively. Additional bands in the spectra are assigned to vibronic transitions of H3CCHCCH+ through quantum chemical calculations and comparison of the spectra with those of similar molecules, the propargyl (C3H3+) and methyl propargyl (H3C4H2+) cations. Excitation of H3CCHCCH+ produces C2H3+ +C2H2 (protonated acetylene + acetylene) and C4H3+ +H2 (protonated diacetylene + hydrogen molecule) photofragments. The preferred formation of C2H3+ +C2H2 photofragments is correctly predicted by master equation simulations, generated from calculated dissociation pathways on the ground state C4H5+ potential energy surface. Electronic spectra of Ar- and N2-tagged C6H4+ cations, generated from acetylene ion-molecule reactions, are recorded over the 265-700 nm range by monitoring C6H4+ and C4H2+ photofragments. Bands are assigned to transitions of five isomers based on previous spectroscopic studies, calculated stationary points on the ground state C6H4+ potential energy surface, hole burning experiments, and spectral simulations generated from TD-DFT calculations. In the C6H4+-Ar spectrum, bands over the 526-700 nm range arise from the B2A’’<-- X2A’’ electronic transition of the 1-hexene-1,3-diyne cation (CH2CHCCCCH+) and the C2Bg <-- X2Au electronic transition of the trans-3-hexene-1,5-diyne cation (HCCCHCHCCH+), with origin transitions occurring at 604 and 581 nm, respectively. Over the 375-460 nm range, broad bands at 440, 425, and 423 nm are assigned to vibronic transitions within the C2A’’<-- X2A’’ band system of the 1-ethynyl-3-methylene-cyclopropa-1,2-diene cation (H2C[c –C3H]CCH+). Sharper bands at 416, 407, and 400 nm are ascribed to the B2A’’<-- X2A’’ electronic transition of the propargyl cyclopropene cation ([c –C3H2]CHCCH+). Between 265 and 375 nm, several bands appear, superimposed on a broad band that extends for several thousand wavenumbers. The 371 nm band is assigned to the B2A’’<-- X2A’’ origin transition of the 1-hexene-1,3-diyne cation (CH2CHCCCCH+), agreeing with previous assignments. In light of the TD-DFT calculations, the bands at 365 and 357 nm are tentatively reassigned to the C2B<--X2B electronic transition of the propadienylidene cyclopropene ([c –C3H2]CCCH2+) isomer. The underlying band possibly arises from the C2Bg <-- X2Au electronic transition of the trans-3-hexene-1,5-diyne (HCCCHCHCCH+) cation, which undergoes a reduction in symmetry upon photoexcitation. Formation of C4H2+ photofragments at wavelengths below 426 nm (2.91 eV) agrees with calculated C6H4+ dissociation pathways on the ground state C6H4+ potential energy surface

On the possible contribution of cationic oxygenated carbon chains C n O+, HC n O+, and OC n O+ ( n = 4 − 9) to the diffuse interstellar bands

International audienceAbstract Only 4 of the diffuse interstellar bands (DIBs) are currently accounted for, ascribed to electronic transitions of C$_{60}^+$. Investigations into carriers of other DIBs historically focus on charged and neutral hydrocarbons, and little information is available regarding oxygenated carbon and hydrocarbon species that result from the two most abundant heavy elements in the interstellar medium, C and O. In this study, we assess whether CnO+, HCnO+, and OCnO+ (n = 4 − 9) cations are viable candidates to account for DIBs using both density-functional theory (DFT) and coupled cluster single-double and perturbative triple theory, CCSD(T). For these species, the linear structures are the most stable isomers with the lowest dissociation threshold corresponding to CO loss. Optical absorptions of the oxygenated carbon chain cations are characterized by calculated vertical excitation wavelengths and their corresponding oscillator strengths using the equation-of-motion CCSD (EOM-CCSD) method. Aside from HC4O+ and HC2n + 1O+, all of the species considered in this study have calculated electronic transitions that lie in the visible or near-infrared spectral regions. Minimal column densities necessary for these cations to account for DIBs have been estimated. Based on present results and the known column densities for neutral oxygenated carbon chains in TMC-1, the growth rate of charged O-bearing carbon chains via ion-neutral reaction mechanisms is judged to be too low to form a sufficient population to give rise to DIBs

Electronic spectrum of the protonated diacetylene cation (H2C4H+)

International audienceTheHB1A1 HX1A1 electronic band system of the protonated diacetylene cation (H2C4H+) is measured over the 230–295 nm range by photodissociating H2C4H+ ions stored in a cryogenic iontrap and by photodissociating H2C4H+ tagged with Ar and N2 in a tandem mass spectrometer. The HB 1A1 HX1A1 band system has an origin at 34 941 cm1 for H2C4H+, 34 934 cm1 for H2C4H+–Ar,and 34 920 cm1 for H2C4H+–N2. The spectra of H2C4H+, H2C4H+–Ar, and H2C4H+–N2 display similar vibronic structure, which is assigned using ab initio calculations to progressions in two symmetric a1 C–C stretch vibrational modes (6 and 4), with band spacings of 860 and 1481 cm1, respectively

New light on the imbroglio surrounding the C8H\u3csup\u3e+\u3c/sup\u3e6 isomers formed from ionized azulene and naphthalene using ion-molecule reactions

Most polycyclic aromatic hydrocarbons (PAHs) can isomerize with internal energies near to or below the dissociation threshold. The C10H+8 group of ions, made up of the naphthalene (Naph+) and the azulene (Azu+) radical cations, is a prototypical example. C8H+6 isomers are important species in the growth kinetics and formation of complex organic molecules, and more generally fragments from larger PAHs, yet information about C8H+6 structures is scarce and contradictory. Here, ion-molecule reactions were carried out and the tunable photoionization chemical monitoring technique was used to probe the C8H+6 isomers formed upon C2H2-loss from Naph+ and Azu+. The experimental findings were interpreted with the support of ab initio and kinetics calculations. To facilitate the interpretation of these data, chemical reactivity starting from phenylacetylene (PA) was studied. It was found that most of the C8H+6 ions formed from C10H8, in a timescale of 40 μs, are PA+ in the vicinity of the dissociation threshold. No evidence of the pentalene radical cation (PE+) was observed and explanations to reconcile previous results are presented

Photo-induced 6π-electrocyclisation and cycloreversion of isolated dithienylethene anions

The diarylethene chromophore is commonly used in light-triggered molecular switches. The chromophore undergoes reversible 6π-electrocyclisation (ring closing) and cycloreversion (ring opening) reactions upon exposure to UV and visible light, respectively, providing bidirectional photoswitching. Here, we investigate the gas-phase photoisomerisation of meta- (m) and para- (p) substituted dithienylethene carboxylate anions (DTE-) using tandem ion mobility mass spectrometry coupled with laser excitation. For p-DTE-, photoisomerisation action spectra associated with 6π-electrocyclisation and cycloreversion processes show band maxima at 300 nm and 615 nm, respectively. In contrast, only the cycloreversion reaction was observed for gas-phase m-DTE- with a maximum response in the action spectrum at 590 nm, highlighting the importance of the carboxylate substitution position on the photochemical properties of DTE molecules. We also observe no evidence for the formation of the undesirable cyclic byproduct in the gas phase, which in solution causes fatigue of the photoswitch

Low Temperature Chlorine-Initiated Oxidation of Small-Chain Methyl Esters: Quantification of Chain-Terminating HO₂‑Elimination Channels

Cl-initiated oxidation reactions of three small-chain methyl esters, methyl propanoate (CH₃CH₂COOCH₃; MP), methyl butanoate (CH₃CH₂CH₂COOCH₃; MB), and methyl valerate (CH₃CH₂CH₂CH₂COOCH₃; MV), are studied at 1 or 8 Torr and 550 and 650 K. Products are monitored as a function of mass, time, and photoionization energy using multiplexed photoionization mass spectrometry coupled to tunable synchrotron photoionization radiation. Pulsed photolysis of molecular chlorine is the source of Cl radicals, which remove an H atom from the ester, forming a free radical. In each case, after addition of O₂ to the initial radicals, chain-terminating HO₂-elimination reactions are observed to be important. Branching ratios among competing HO₂-elimination channels are determined via absolute photoionization spectra of the unsaturated methyl ester coproducts. At 550 K, HO₂-elimination is observed to be selective, resulting in nearly exclusive production of the conjugated methyl ester coproducts, methyl propenoate, methyl-2-butenoate, and methyl-2-pentenoate, respectively. However, in MV, upon raising the temperature to 650 K, other HO₂-elimination pathways are observed that yield methyl-3-pentenoate and methyl-4-pentenoate. In each methyl ester oxidation reaction, a peak is observed at a mass consistent with cyclic ether formation, indicating chain-propagating OH loss/ring formation pathways via QOOH intermediates. Evidence is observed for the participation of resonance-stabilized QOOH in the most prominent cyclic ether pathways. Stationary point energies for HO₂-elimination pathways and select cyclic ether formation channels are calculated at the CBS-QB3 level of theory and assist in the assignment of reaction pathways and final products