Ab initio molecular orbital study of the effects of basis set size on the calculated structure and acidity of hydroxyl groups in framework molecular sieves

Journal Article · Thu Dec 10 04:00:00 EST 1992 · Journal of Physical Chemistry; (United States)

DOI:https://doi.org/10.1021/j100204a029· OSTI ID:6594930

Nicholas, J B; Hopfinger, A J ^[1]; Harrison, R J; Iton, L E; Curtiss, L A; Winans, R E ^[2]

---

1. Univ. of Illinois, Chicago (United States)
2. Argonne National Lab., IL (United States)

The structures, force constants, and relative acidities of a series of molecules that mimic the geometries of terminal and bridging hydroxyl groups in various substituted zeolites and clays are calculated by ab initio molecular orbital methods. The molecules are structural analogs of disiloxane H[sub 3]T-O-TH[sub 3], and the protonated form H[sub 3]T, Si, Al, B, and P. Also included are H[sub 3]/SiO[sup [minus]] and H[sub 3]SiOH that mimic terminal hydroxyl groups that occur at the zeolite surface and defect sites. Results are presented for restricted Hartree-Fock (RHF) calculations at levels of theory that range from the minimal RHF/STO-3G to double- and triple-zeta basis sets with multiple polarization functions. The study shows that the structures of the molecules converge toward limiting values at the higher levels of theory. The theoretical trend in acidity of the terminal and bridging hydroxyls, as determined by the proton affinity, the length of the O-H bond, the charge on the hydroxyl proton, and the O-H stretching force constants and frequencies, is presented and is in agreement with experimental determinations. The calculated proton affinity is also shown to depend on the basis set size; however, the acidity trend is well reproduced at all levels of theory except RHF/STO-3G. 38 refs., 7 figs., 15 tabs.

• DOE Contract Number: W-31109-ENG-38
• OSTI ID: 6594930
• Journal Information: Journal of Physical Chemistry; (United States), Journal Name: Journal of Physical Chemistry; (United States) Vol. 96:25; ISSN JPCHAX; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020400* -- Petroleum-- Processing
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201 -- Chemical & Physicochemical Properties
ADSORBENTS
CALCULATION METHODS
CLAYS
DATA
HARTREE-FOCK METHOD
INFORMATION
INORGANIC ION EXCHANGERS
ION EXCHANGE MATERIALS
MATERIALS
MATHEMATICAL MODELS
MINERALS
MOLECULAR MODELS
MOLECULAR ORBITAL METHOD
MOLECULAR SIEVES
NUMERICAL DATA
PH VALUE
SILICATE MINERALS
THEORETICAL DATA
ZEOLITES