Ab initio Studies of NH₄⁺(H₂O)_1-5 and the Influence of Hydrogen-Bonding Nonadditivity on Geometries and Vibrations

Abstract

Ab initio calculations were performed to investigate the structures, energetics, and vibrations of NH4 +(H2O) n cluster ions at n = 0−5. Equilibrium geometries of NH4 + and NH4 +−H2O are optimized at the MP2, MP4, CCD, QCISD, and B3LYP levels using the 6-31G*, 6-31G**, 6-31+G*, 6-31++G**, 6-311+G**, and 6-311++G** basis sets. The benchmark calculations indicate that using MP2 and B3LYP approaches with the 6-31+G* basis set is well suited for characterizing large NH4 +(H2O) n clusters. The two approaches correspondingly find the existence of a number of structural isomers at n = 2−5, of which the isomer with a filled first solvation shell is lowest in energy at n = 4. The calculations further predict that, at n = 5, the lowest energy isomer contains a four-membered ring with the second-shell H2O acting as a double-proton acceptor (AA). The prediction is in good agreement with the observation of jet-cooled NH4 +(H2O)5, where a characteristic hydrogen-bonded-OH stretching absorption at ∼3550 cm-1 is identified for the AA−H2O molecule in the vibrational predissociation spectra (Wang, Y.-S.; Chang, H.-C.; Jiang, J. C.; Lin, S. H.; Lee, Y. T.; Chang, H.-C. J. Am. Chem. Soc. 1998, 10 2, 8777). In this study, in addition to energetics, how hydrogen-bonding nonadditivity influences the geometries and vibrations of these clusters is analyzed.

Solvation processes in steam: Ab initio calculations of ion–solvent structures and clustering equilibria

“…4 are (1) the good agreement in entropy trends between our work and those from HPMS experiments by Meot-Ner and coworkers as well as results from a combined density functional (B3LYP) and ab initio study (MP2-level) reported by Jiang et al (1999) (Note, entropy changes have been calculated from Jiangs' reported MP2-level hydration enthalpies and free energies up to m = 5 and added (2) the excellent agreement between our calculated and reported (Meot-Ner, 1984) incremental hydration enthalpies and entropies up to the NH 4 þ ÁðH 2 OÞ 6 cluster. A key feature that emerges from recent IR-studies of NH 4 þ ÁðH 2 OÞ n clusters by Chang et al (1998) and Jiang et al (1999) is a strong temperature dependence of the ring opening and closing process in NH 4 þ ÁðH 2 OÞ 5 (f5) and NH 4 þ ÁðH 2 OÞ 6 (f6). The closure toward the ring structures f5 and f6 as shown in Fig.…”

“…Experimental clustering equilibria for NH 4 þ ÁðH 2 OÞ m and NH 4 þ ÁðH 2 SÞ n are taken from HPMS studies by Meot-Ner and Speller (1986), Meot-Ner (1984), Payzant et al (1973) and Meot-Ner and Sieck (1985), respectively. Our discussion also includes two recent combined IR-spectroscopic and ab initio studies of ammonium-water clusters by Wang et al (1998) and Jiang et al (1999).…”

“…Ab initio calculations of clustering equilibria for the above hydration reactions can provide detailed insight into the nature of proton-hydration in high-temperature, low-density aqueous systems containing a variety of components such as H 2 S, H 3 S + and NH 4 þ . Over the last three decades, numerous quantum chemical studies have been conducted, that have focused on the thermochemistry of stepwise solvation of the hydroniumion (Newton, 1977;Yamabe et al, 1985a;Lee et al, 1990;Lee and Dyke, 1991;Xie et al, 1994;Corongiu et al, 1995;Kochanski et al, 1997;Jiang et al, 2000;Mella and Clary, 2003;Lee et al, 2004;Mella et al, 2005;Pickard et al, 2005b), the ammonium ion (Kassab et al, 1990;Armstrong et al, 1993;Bueker and Uggerud, 1995;Contador et al, 1997;Wang et al, 1998;Jiang et al, 1999;Pickard et al, 2005a;Intharathep et al, 2005) with water as well as on the stability of sulfonium clusters with H 2 S as a solvent (Yamabe et al, 1985b;Ligon and Gäb, 2005).…”

“…NH 4 þ ÁðH 2 OÞ 2-5 , Armstrong et al, 1993), complete basis set CBS-QB3 and CBS-QCI/ APNO calculations for NH 4 þ ÁðH 2 OÞ 1-4 (Pickard et al, 2005a), B3LYP density functional calculations by Parrinello and coworkers NH 4 þ ÁðH 2 OÞ 1-4 (Brugé et al, 1999) and the combined ab initio and density functional theory work for NH 4 þ ÁðH 2 OÞ 1-5 by Jiang et al (1999). All above theoretical studies in the NH 4 þ ÁðH 2 OÞ n system yielded hydration energies that were in good agreement with experimental clustering studies (Meot-Ner, 1984).…”

Pressure and temperature dependence on the hydrogen bonding and dynamics of ammonium ion in liquid water: A molecular dynamics simulations study

“…Also, in many natural and agricultural ecosystems, NH 4 + ion is the predominant nitrogen source and an important micronutrient in the world's water system [1]. Hence, a detailed understanding of the solvation process of ammonium ion plays an important role both in chemistry and biology and the subject of extensive experimental [2][3][4][5][6][7][8][9][10] and theoretical [11][12][13][14][15][16][17][18][19][20] studies. The faster rotational dynamics of NH 4 + in water is a central focus of these studies.…”