We have carried out a natural bond orbital analysis of hydrogen bonding in the water dimer for the near‐Hartree–Fock wave function of Popkie, Kistenmacher, and Clementi, extending previous studies based on smaller basis sets and less realistic geometry. We find that interactions which may properly be described as ‘‘charge transfer’’ (particularly the n‐σ*OH interaction along the H‐bond axis) play a critical role in the formation of the hydrogen bond, and without these interactions the water dimer would be 3–5 kcal/mol repulsive at the observed equilibrium distance. We discuss this result in relationship to Klemperer’s general picture of the bonding in van der Waals molecules, and to previous theoretical analyses of hydrogen bonding by the method of Kitaura and Morokuma.

Natural bond orbital analysis of near‐Hartree–Fock water dimer

We describe the design, construction, and operation of a new type of microwave spectrograph which allows the measurement of the resonant transitions of transient or otherwise short‐lived species. The spectrograph is composed of three parts: a Fabry–Perot cavity, a pulsed supersonic nozzle as a source for the sample, and the pulsed microwave Fourier transform method. Following a detailed discussion of the three above components in the spectrograph, the operation of the entire system is described and several examples are given.

Fabry–Perot cavity pulsed Fourier transform microwave spectrometer with a pulsed nozzle particle source

The term "hydrogen bond" has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material sci- entists, there has been a continual debate about what this term means. This debate has inten- sified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X-HY hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some char- acteristics that are observed in typical hydrogen-bonding environments.

https://www.degruyter.com/downloadpdf/j/pac.2011.83.issue-8/pac-rep-10-01-01/pac-rep-10-01-01.xml

Defining the hydrogen bond: An account (IUPAC Technical Report)

Designs for a broadband chirped pulse Fourier transform microwave (CP-FTMW) spectrometer are presented. The spectrometer is capable of measuring the 7-18 GHz region of a rotational spectrum in a single data acquisition. One design uses a 4.2 Gsampless arbitrary waveform generator (AWG) to produce a 1 mus duration chirped pulse with a linear frequency sweep of 1.375 GHz. This pulse is sent through a microwave circuit to multiply the bandwidth of the pulse by a factor of 8 and upconvert it to the 7.5-18.5 GHz range. The chirped pulse is amplified by a traveling wave tube amplifier and broadcast inside the spectrometer by using a double ridge standard gain horn antenna. The broadband molecular free induction decay (FID) is received by a second horn antenna, downconverted, and digitized by a 40 Gsampless (12 GHz hardware bandwidth) digital oscilloscope. The second design uses a simplified pulse generation and FID detection scheme, employing current state-of-the-art high-speed digital electronics. In this spectrometer, a chirped pulse with 12 GHz of bandwidth is directly generated by using a 20 Gsampless AWG and upconverted in a single step with an ultrabroadband mixer. The amplified molecular emission is directly detected by using a 50 Gsampless digital oscilloscope with 18 GHz bandwidth. In both designs, fast Fourier transform of the FID produces the frequency domain rotational spectrum in the 7-18 GHz range. The performance of the CP-FTMW spectrometer is compared to a Balle-Flygare-type cavity-FTMW spectrometer. The CP-FTMW spectrometer produces an equal sensitivity spectrum with a factor of 40 reduction in measurement time and a reduction in sample consumption by a factor of 20. The CP-FTMW spectrometer also displays good intensity accuracy for both sample number density and rotational transition moment. Strategies to reduce the CP-FTMW measurement time by another factor of 90 while simultaneously reducing the sample consumption by a factor of 30 are demonstrated.

A broadband Fourier transform microwave spectrometer based on chirped pulse excitation

The design, performance, and operation of a broadband (3–26.5 GHz) high resolution microwave spectrometer is described. In comparison to previously developed molecular beam Fabry–Perot resonator spectrometers the design presented here implements some significant improvements: a coaxially oriented beam resonator arrangement (COBRA) formed by a confocal pair of mirrors incorporating an electromechanical valve and employing two pairs of microwave antennas, and a multioctave Fourier‐transform microwave (FTMW) instrument providing the pulsed excitation source with microwave pulse phase‐inversion scheme and the low‐noise receiving system employing image‐rejection downconversion with superheterodyne as well as quadrature detection. The entire apparatus, fully automated for scanning operation, covers a frequency range of more than three octaves. The novel design of the FTMW instrument does not require any changes of the spectrometer hardware in order to reach all regions of its spectral range. While operated in h...

A multioctave coaxially oriented beam‐resonator arrangement Fourier‐transform microwave spectrometer

The vibrational ground state rotational spectroscopic constants and structure of the HCN dimer

The rotational spectrum of the ArDF van der Waals molecule has been assigned using pulsed Fourier transform microwave spectroscopy in a Fabry‐Perot cavity with a pulsed supersonic nozzle as the molecular source. The DF nuclear spin–nuclear spin and D nuclear quadrupole coupling constants are 12.5(11) and 194.7(20) kHz, respectively. These constants yield equivalent structural information and are used together with the rotational constant to obtain the structure of ArDF. The rotational spectrum of ArHF was also re‐examined and the HF spin–spin constant found to be 49.0(19) kHz. The structure of ArHF was then obtained from the rotational and HF spin–spin constants. Finally, the vibrationally averaged angles in 14 complexes of the type X–H(D)Y (X=Ar, Kr, or Xe; Y=F, Cl, or Br) are calculated using a multipole potential and compared with experiment. The agreement between the observed and calculated angles is very good.

Molecular structure of ArDF: An analysis of the bending mode in the rare gas–hydrogen halides

Rotational spectra have been observed for four isotopes of ArHBr and ArDBr and eight isotopes of KrHBr and KrDBr using a Fabry–Perot Fourier transform spectrometer with a pulsed supersonic nozzle as the molecular source. The rotational constants in the ground vibrational state ?0 with their centrifugal distortions DJ, as well as Br nuclear quadrupole coupling constants χa, are given. In addition, an important centrifugal distortion of the Br quadrupole coupling constant, Dχ, an indicator of the coupling between the radial and angular potentials, is given for ArHBr and KrHBr. The Br spin–rotation interaction c in ArHBr is also obtained. The results are: The molecular structures are consistent with a linear equilibrium geometry with the H(D) atom located between Br and the rare gas atoms. The complexes undergo large amplitude vibrations and estimates of the bending and stretching force constants and frequencies are given. By combining the bending, stretching, and their coupling, we have obtained the harmoni...

Rotational spectra and molecular structures of ArHBr and KrHBr

The gas dynamics of a pulsed supersonic nozzle molecular source are investigated by using a pulsed Fabry–Perot cavity microwave spectrometer to obtain free induction decay signals from rotational two‐level systems in the gas expansion. An equation is derived giving the time domain emission signal line shape as an integral over the active molecular distribution in the beam. The Doppler splitting phenomenon is discussed in detail. Experimental line shapes are deconvoluted to give molecular velocities, dephasing times, and density distributions. We find that the density distribution of active molecules from the the pulsed nozzle varies rapidly in time, starting with a depletion on the nozzle axis at short times after the nozzle is opened, and changing to on‐axis concentration at longer times. Results obtained with the gas nozzle axis oriented at angles ranging from 0 ° to 90 ° with respect to the direction of propagation of the microwaves are reported.

The gas dynamics of a pulsed supersonic nozzle molecular source as observed with a Fabry–Perot cavity microwave spectrometer

A semiclassical theory has been developed to describe pulsed Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity. A density matrix formalism is used to study the interaction of a two‐level quantum system with a classical standing wave electric field, appropriate for the Fabry–Perot cavity. Equations describing the polarization of, and subsequent emission of radiation by arbitrary distributions of molecules in the cavity are derived. The specific problem of a static Maxwell–Boltzmann gas is studied in detail, both theoretically and experimentally. The static gas line shape in the power‐broadened limit is described by an ordinary Doppler and pressure broadened envelope. Sensitivities of the ordinary waveguide cell and Fabry–Perot cavity pulsed Fourier transform spectrometers using static gas samples are compared.

The theory of pulsed Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity: Static gas

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