Yongjun Hu

Competitive fragmentation pathways of acetic acid dimer explored by synchrotron VUV photoionization mass spectrometry and electronic structure calculations

In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH(3)COOH)(n)·H(+), the feature related to the fragment ions (CH(3)COOH)H(+)·COO (105 amu) via β-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH(3)COOH)·H(+) and (CH(3)COOH)H(+)·COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH(3)COOH)H(+)·COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH(3)COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH(3)COOH)·CH(3)CO(+). Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.

Conformational equilibrium and hydrogen bonding in liquid 2-phenylethylamine explored by Raman spectroscopy and theoretical calculations.

2-Phenylethylamine (PEA) is the simplest aromatic amine neurotransmitter, as well as one of the most important. In this work, the conformational equilibrium and hydrogen bonding in liquid PEA were studied by means of Raman spectroscopy and theoretical calculations (DFT/MP2). By changing the orientation of the ethyl and the NH(2) group, nine possible conformers of PEA were found, including four degenerate conformers. Comparison of the experimental Raman spectra of liquid PEA and the calculated Raman spectra of the five typical conformers in selected regions (550-800 and 1250-1500 cm(-1)) revealed that the five conformers can coexist in conformational equilibrium in the liquid. The NH(2) stretching mode of the liquid is red-shifted by ca. 30 cm(-1) relative to that of an isolated PEA molecule (measured previously), implying that intermolecular N-H···N hydrogen bonds play an important role in liquid PEA. The relative intensity of the Raman band at 762 cm(-1) was found to increase with increasing temperature, indicating that the anti conformer might be favorable in liquid PEA at room temperature. The blue shift of the band for the bonded N-H stretch with increasing temperature also provides evidence of the existence of intermolecular N-H···N hydrogen bonds.

Site-selective ionization of ethanol dimer under the tunable synchrotron VUV radiation and its subsequent fragmentation.

Site-selective ionization of ethanol dimer and the subsequent fragmentation were studied by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. With photoionization efficiency spectra measurements and theoretical calculations, the detailed mechanisms of the ionization-dissociation processes of ethanol dimer under VUV irradiation were explored. In 9.49-10.89 eV photon energy range, it was found that the ejection of the highest occupied molecular orbital (HOMO) electron from hydrogen bond donor induces a rapid barrierless proton-transfer process followed by two competitive dissociation channels, generating (C2H5OH)[middle dot]H(+) and CH2O[middle dot](C2H5OH)H(+), respectively. The latter comes from a carbon-carbon bond cleavage in the donor. While the photon energy is 10.9-11.58 eV, the electron of HOMO-1 of the hydrogen bond acceptor, is removed. Besides the dissociation channel to produce C2H5OH and C2H5OH(+), a new channel to generate (C2H5OH)[middle dot]CH2OH(+) is opened, where the cleavage of the carbon-carbon bond occurs in the acceptor. When the photon energy increases to 11.58 eV, the electron from HOMO-2 is ejected.

The pH dependent Raman spectroscopic study of caffeine.

First of all the surface enhanced Raman spectroscopy (SERS) and normal Raman spectra of caffeine aqueous solution were obtained at different pH values. In order to obtain the detailed vibrational assignments of the Raman spectroscopy, the geometry of caffeine molecule was optimized by density functional theory (DFT) calculation. By comparing the SERS of caffeine with its normal spectra at different pH values; it is concluded that pH value can dramatically affect the SERS of caffeine, but barely affect the normal Raman spectrum of caffeine aqueous solution. It can essentially affect the reorientation of caffeine molecule to the Ag colloid surface, but cannot impact the vibration of functional groups and chemical bonds in caffeine molecule.

Conformations of 2-thiouracil in the aqueous solution and its adsorption behavior on the gold substrates explored by DFT calculations and experimental methods

2-Thiouracil, a thio-derivative of uracil, may appear in various tautomeric forms due to the different positions of protons. In this paper, the adsorption behavior and conformations of 2-thiouracil on the gold substrates are inspected by means of surface-enhanced Raman scattering (SERS) and density functional theory (DFT) calculations. The results indicate that all the enhanced bands are assigned to in-plane vibration modes. Besides, most of the bands related to N and S atoms are significantly enhanced and have obvious shifts in the SERS spectra. Furthermore, the CO stretching band at 1695 cm(-1) also appears in the SERS spectra. The theoretical SERS spectra of 2TU-Au₄ and 2TU2-Au₄ complexes agree well with the experimental SERS spectra of 2-thiouracil at 0.04 mM. Meanwhile, we calculate that the binding energies for 2TU2-Au₄ and 2TU-Au₄ are ca. 70 and 50 kcal/mol, respectively. Those results above imply that the 2TU and 2TU2 conformers can exist stably in the aqueous solution and both of them are vertically chemisorbed on the gold surfaces. For the 2TU, it is adsorbed on the gold surfaces through N1H position and the sulfur atom. While the 2TU2 adsorbed on the gold substrates through the N1 site and its deprotonated sulfur atom.

Gas-phase conformational preference of the smallest saccharide (glycolaldehyde) and its hydrated complexes with bridged hydrogen bonding

Glycoaldehyde (GA, HOCH2CHO) is the simplest sugar unit of the carbohydrates and the only sugar to have been detected in interstellar space to date. In the present report, the conformation of GA and its flexible hydrated complexes have been investigated in the gas phase by using mass-selected infrared (IR) spectroscopy based on vacuum-ultraviolet single photon ionization (118 nm). With the aid of theoretical calculations, the neutral GA bearing a ring-type intramolecular hydrogen bonding interaction was confirmed to be the dominant isomer in the gas phase. Moreover, the water molecules in the monohydrated complexes preferentially broke the intramolecular hydrogen bond and bridged the carbonyl oxygen and hydroxyl hydrogen of GA with two additional intermolecular H-bonds, revealing the “working rules” governing preferred binding. The theoretical results confirmed that the existence probability of the two lowest energy conformations stabilized by two intermolecular hydrogen bonds would be larger than that of the next two isomers with one intramolecular plus one intermolecular hydrogen bond. Structural investigation of hydrated GA conformers has revealed that the water molecules play the role of a bridge through intermolecular H-bonds, achieving selective population of specific GA molecular conformations. These results suggest that these hydrogen-bonded bridge structures in the hydrated complexes may provide good models for recognition in larger systems.

Molecular structures of gas-phase neutral morpholine and its monohydrated complexes: experimental and theoretical approaches

Morpholine (NH(CH2CH2)2O) is a typical six-membered aliphatic heterocyclic compound. Herein, infrared plus vacuum ultraviolet (IR/VUV) single photon “soft” ionization spectroscopy was employed to study the structures of neutral morpholine and its monohydrated clusters. Theoretical calculations revealed that the structures containing equatorial-chair and axial-chair conformations were the most stable conformers in the gas phase, and this was confirmed by IR spectral analysis. Analysis of the observed and calculated spectra of the monohydrated clusters suggested that multiple conformers may co-exist in the molecular beam, and that the water molecule acts as a hydrogen donor. In the most stable structure, the hydrogen atom of the water molecule is bound to the NH group of the equatorial-chair conformer of morpholine. Moreover, the water molecules simultaneously serve as hydrogen bond donors for the NH group and interact with the CH group weakly. It is suggested that the weak intermolecular CH⋯O interaction is also responsible for the molecular stability.

Dominant conformer of tetrahydropyran-2-methanol and its clusters in the gas phase explored by the use of VUV photoionization and vibrational spectroscopy

Tetrahydropyran-2-methanol (THPM) is a typical alcohol containing a six-member cyclic ether, which can be considered as the model molecule of cyclic sugar. Herein, vacuum ultraviolet (VUV) photodissociation spectroscopy is employed to study fragmentation pathways and infrared (IR) plus VUV photoionization spectroscopy to investigate the structures of neutral THPM and its clusters with the size up to the trimer. Qualitative structural assignments are confirmed for the neutral species and ions based on MP2/aug-cc-pVTZ and ωB97X-D/cc-pVTZ calculations. The fragment cations at m/z = 84, 85, and 98 arise by the losing of CH2OH, CH3OH, and H2O from the monomer, respectively, as a result of C-C bond and C-O bond dissociation under the VUV (118 nm) radiation. It is found that the loss of CH3OH and H2O involves hydrogen transfer from the CH2 group to the dissociating CH2 and OH groups. Comparing the observed and calculated spectra of the monomer THPM, it suggests that the conformer containing a chair tetrahydropyran ring and an intramolecular hydrogen bond would be dominantly survived in a supersonic beam. Moreover, the IR spectra of larger clusters n > 1 (n = 2, 3) show only the broad hydrogen bonded OH stretch mode, and thus these larger clusters would form a closed-cyclic structure, where all OH groups are participating in hydrogen bonding. Partially the CH stretch positions of THPM clusters do not change significantly with the increasing of cluster size, thus the CH and CH2 groups are not involved in H-bonding interactions.

Laser Desorption Postionization Mass Spectrometry Imaging of Folic Acid Molecules in Tumor Tissue

Mass spectrometry imaging (MSI) is an innovative and powerful tool in biomedical research. It is well-known that folic acid (FA) has a high affinity for folic acid receptor (FR), which is overexpressing in epithelial cancer. Herein, we propose a novel method to diagnose cancer through direct mapping of the label-free FA spatial distribution in tissue sections by state-of-the-art laser desorption postionization-mass spectrometry imaging (LDPI-MSI). Compared with other tumor imaging methods, such as fluorescence imaging, photoacoustic imaging (PAI), magnetic resonance imaging (MRI), and micro-SPECT/CT, complicated synthesis and labeling processes are not required. The LDPI-MSI was performed on 30 μm thick sections from a murine model of breast cancer (inoculation of 4T1 cells) that were predosed with 20 mg/kg of FA. The image obtained from the characteristic mass spectrometric signature of FA at m/z 265 illustrated that FA was concentrated primarily in tumor tissue and displayed somewhat lower retention in adjacent normal controls. The results suggest that the proposed method could be used potentially in cancer diagnosis.

Boron nitride nanotubes matrix for signal enhancement in infrared laser desorption postionization mass spectrometry

The nanomaterials function as the substrate to trap analytes, absorb energy from the laser irradiation and transfer energy to the analytes to facilitate the laser desorption process. In this work, the signal intensity and reproducibility of analytes with nanomaterials as matrices were explored by laser desorption postionization mass spectrometry (LDPI-MS). Herein, the desorbed neutral species were further ionized by vacuum ultraviolet (VUV, 118 nm) and analyzed by mass spectrometer. Compared with other nanomaterial matrices such as graphene and carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs) exhibited much higher desorption efficiency under infrared (IR) light and produced no background signal in the whole mass range by LDPI-MS. Additionally, this method was successfully and firstly exploited to in situ detection and imaging for drugs of low concentration in intact tissues, which proved the utility, facility and convenience of this method applied in drug discovery and biomedical research.