Ammonia dimer: Further structural studies

An Exploration of the Hydrogen Bond Donor Ability of Ammonia

Ammonia is an important molecule due to its wide use in the fertiliser industry. It is also used in aminolysis reactions. Theoretical studies of the reaction mechanism predict that in reactive complexes and transition states, ammonia acts as a hydrogen bond donor forming N-H⋅⋅⋅O hydrogen bond. Experimental reports of N-H⋅⋅⋅O hydrogen bond, where ammonia acts as a hydrogen bond donor are scarce. Herein, the hydrogen bond donor ability of ammonia is investigated with three chalcogen atoms i. e. O, S, and Se using matrix isolation infrared spectroscopy and electronic structure calculations. In addition, the chalcogen bond acceptor ability of ammonia has also been investigated. The hydrogen bond acceptor molecules used here are O(CH3 )2 , S(CH3 )2 , and Se(CH3 )2 . The formation of the 1 : 1 complex has been monitored in the N-H symmetric and anti-symmetric stretching modes of ammonia. The nature of the complex has been delineated using Atoms in Molecules analysis, Natural Bond Orbital analysis, and Energy Decomposition Analysis. This work presents the first comparison of the hydrogen bond donor ability of ammonia with O, S, and Se.

Ammonia dimer: extremely fluxional but still hydrogen bonded

In the 1980s, Nelson, Fraser, and Klemperer (NFK) published an experimentally derived structure of the ammonia dimer dramatically different from the structure determined computationally, which led these authors to the question "Does ammonia hydrogen bond?". This question has not yet been answered satisfactorily. To answer it, we have developed an ab initio potential energy surface (PES) for this dimer at the limits of the current computational capabilities and performed essentially exact six-dimensional calculations of the vibration-rotation-tunneling (VRT) spectra of NH₃-NH₃ and ND₃-ND₃, obtaining an unprecedented agreement with experimental spectra. In agreement with other recent electronic structure calculations, the global minimum on the PES is in a substantially bent hydrogen-bonded configuration. Since the bottom of the PES is exceptionally flat, the dimer is extremely fluxional and the probability of finding it in configurations that are not hydrogen bonded is high. Nevertheless, the probability of hydrogen-bonded configurations is large enough to consider the ammonia dimer to be hydrogen bonded. We also show that NFK's inference that the ammonia dimer is nearly rigid actually results from unusual cancellations between quantum effects that generate differences in spectra of different isotopologues.

A full-dimensional ab initio potential energy and dipole moment surfaces for (NH3)2

A full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) for the ammonia dimer (NH₃)₂ are reported. The database of the PES consists of 27 736 ab initio energy points and all of these points were calculated at the UCCSD(T)-F12a/AVTZ level. The PES was fitted by using the fundamental invariant neural network (FI-NN) method that satisfies the permutational symmetry of identical atoms, and the root mean square fitting error for the PES is very small as low as 0.562 meV. The geometries for the (NH₃)₂ DMS are the same as those used for the PES and are calculated at the XYG3/AVTZ level. This PES can describe a variety of internal floppy motions, including all kinds of vibrational modes no matter intermolecular or intramolecular. The CCSD(T)-PES can dissociate correctly to two NH₃ monomers, with D_e = 1135.55 cm^-1 (13.58 kJ/mol) which agrees accurately with the 13.5 ± 0.3 kJ/mol predicted by previous work.

Structures and relative stabilities of ammonia clusters at different temperatures: DFT vs. ab initio

The global minimum energy structures of (NH₃)_n=2–10are pointed out for the first time at a given temperature.

Evolution of Microwave Spectroscopy at the National Bureau of Standards (NBS) and the National Institute of Standards and Technology (NIST)

This paper describes the beginning and evolution of microwave rotational spectroscopic research starting in 1954 at the National Bureau of Standards (NBS), located at that time in Washington, DC, through the present at NIST in Gaithersburg, MD. David Lide was hired in 1954 to start this research employing Stark modulated waveguide septum cells. When Donald R. Johnson joined the lab in 1968, he developed parallel plate cells coupled with rf and DC discharge methods to study free radicals and transient species. In the mid 1980s Lovas and Suenram constructed a pulsed molecular beam Fourier Transform microwave (FTMW) spectrometer to study hydrogen bonded and van der Waals dimers and trimers. This article describes the types of molecules studied and the type molecular properties derived from these measurements as well as some of the instruments developed for these studies. The two major areas of application described are atmospheric chemistry and molecular radio astronomy.

Terahertz Vibration−Rotation−Tunneling Spectroscopy of the Ammonia Dimer. II. A−E States of an Out-of-Plane Vibration and an In-Plane Vibration

Terahertz vibration-rotation-tunneling transitions have been measured between ca. 78.5 and 91.9 cm-1, and assigned to A-E (ortho-para) combinations of NH3 monomer states. The spectrum is complicated by inversion splittings that correlate to E symmetry monomer rovibronic states. Twenty progressions have been assigned to six excited states involving an out-of-plane vibration and an in-plane intermolecular vibration. The quality of the fit was affected by strong Coriolis interactions among these states and possibly an additional K = 2 state that was not explicitly observed in the data.

Study of NH Stretching Vibrations in Small Ammonia Clusters by Infrared Spectroscopy in He Droplets and ab Initio Calculations†

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the Møller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.

Study of the Structure, Energetics, and Vibrational Properties of Small Ammonia Clusters (NH3)n (n = 2−5) Using Correlated ab Initio Methods

Equilibrium geometries, interaction energies, and harmonic frequencies of (NH3)n isomers (n = 2-5) have been computed using correlated calculations (MP2) in conjunction with Dunning's aug-cc-pVXZ (X = D, T, Q) basis sets and the Counterpoise procedure. Whenever available, literature values for the binding energy and geometry of dimers and trimers agree well with our data. Low lying isomers for (NH3)4 and (NH3)5 have been found to have similar binding energies (roughly 16 and 20 kcal/mol for the tetramer and pentamer, respectively), perhaps suggesting the presence of a very smooth energy landscape. Using BSSE corrected forces or freezing the monomer structure to its gas phase geometry have been found to have only a weak impact on the energetic and structural properties of the clusters. The effect of zero-point energy (ZPE) on the relative stability of the clusters has been estimated using harmonic frequencies. The latter also highlighted the presence of vibrational fingerprints for the presence of double acceptor ammonia molecules. Many-body effects for (NH3)n isomers (n = 2-4) have been investigated to explore the possibility of building a pairwise interaction model for ammonia. In the frame of the work presented, we have found the 3-body effect to account for 10-15% of the total interaction energy, whereas the 4-body effects may be neglected as first approximation.

Terahertz Vibration−Rotation-Tunneling Spectroscopy of the Ammonia Dimer: Characterization of an out of Plane Vibration

The terahertz vibration-rotation-tunneling (VRT) spectrum of the ammonia dimer (NH(3))(2) has been measured between ca. 78.5 and 91.9 cm(-1). The dipole-allowed transitions are separated into three groups that correspond to the 3-fold internal rotation of the NH(3) subunits. Transitions have been assigned for VRT states of the A-A (ortho-ortho) combinations of NH(3) monomer states. The spectrum is further complicated by strong Coriolis interactions. K = 0 <-- 0, K = 1 <-- 0, K = 0 <-- 1, and K = 1 <-- 1 progressions have been assigned. The band origins, rotational constants, asymmetry doubling, centrifugal distortion, and Coriolis coupling constant have been determined from the fit to an effective Hamiltonian. These VRT transitions are tentatively assigned to an out of plane vibration with a K = 0 state at 89.141305(47) cm(-1), and a K = 1 state at 86.77785(9) cm(-1).