J. Chem. Sci. Vol. 127, No. 8, August 2015, pp. 1491–1496. c Indian Academy of Sciences. DOI 10.1007/s12039-015-0857-3 Theoretical study on the molecular and crystal structures of nitrogen trifluoride and it’s adduct with BF3 HONGCHEN DUa,b,c aSchool of Science, Zhejiang A & F University, Linan, 311300, China bZhejiang Provincial Key Laboratory of Chemical Utilization of Forestry Biomass, Zhejiang A & F University, Lin’an, 311300, China cPresent address: Weifang University of Science and Technology, Jinguang Street 1299, Shouguang, 262700, China e-mail: [email protected]; [email protected] MS received 30 April 2014; revised 30 October 2014; accepted 27 January 2015 Abstract. The molecular and crystal structure of the adduct NF3·BF3 was studied computationally using density functional theory. It shows that the adduct exists in the form of a complex but is not ionic. The heats of formation in the gas and the condensed phase of the adduct are −1266.09 and −1276.37 kJ·mol−1, respectively, which indicates that it is stable under atmospheric conditions. The crystal form belongs to P 21/c space group. The calculated large band gap (Eg) of the crystal proves that it is stable. The conduction band (LUCO) is mainly contributed by the p orbital of N atom and the valence band (HOCO) from the p orbital of F atom. Keywords. Molecular; crystal; structure; property; theoretical study. 1. Introduction investigation even though there is limited information on the structure–property relationship, especially on the Molecular complexes containing boron trifluoride as a crystal structure of NF3 with BF3. In 1996, Ford et al. 1 Lewis acid are known for many years. Nitrogen tri- performed a theoretical study on the adduct, mainly fluoride (NF3) is a colourless, toxic, odourless, non- its binding energies, using the ab initio method.11 This flammable gas, which was first prepared in 1928 by paper aims to study the molecular and crystal structure Ruff, Fischer, and Luft2 by electrolyzing molten anhy- of the complex NF ·BF and the properties. drous ammonium difluoride in an electrically heated 3 3 copper cell. The electrolytic process for the preparation 3–6 2. Computational details of NF3 has been improved by a number of workers and has been patented. Nitrogen trifluoride can also be The title compound was optimized at the B3LYP/CC- formed by direct fluorination of ammonia. It is a stable gas with strong oxidizing properties. It can be used as PVTZ level, and vibrational analysis was performed a potential oxidant for spacecraft propulsion. Decades thereafter for the most stable conformer with the Gaus- 12 ago, studies on the compound have been made: the sian 03 program package. Previous studies have infrared spectrum of NF3 has been reported by Bailey, shown that Beck’s three-parameter nonlocal exchange 7 Hale, and Thompson. In 1950, NF3 was shown to have functional along with the Lee-Yang-Parr nonlocal cor- 8,9 the C3v symmetry. relation functional (B3LYP) can figure out the accurate Nitrogen trifluoride is mainly used for cleaning of energy, molecular structure, and vibrational frequency, PECVD chambers in high-volume production of liquid which are very close to the corresponding experimental crystal displays and silicon-based thin film solar cells. results.13–16 NF3 has been considered environmentally preferable ◦ The gas phase heat of formation (fH gas)was substitute for sulfur hexafluorine or perfluorocarbons obtained using the following isodesmic reaction (1): such as hexafluoroethane.10 It proved to be far less reac- tive than other nitrogen trihalides, nitrogen trichloride, NF3 + BF3 = NF3 · BF3 (1) nitrogen tribromide and nitrogen triiodide, all of which The changes in enthalpy (H ◦ ) of the above reac- are explosives. But explosion occurs when nitrogen tri- 298 tions were evaluated using equation (2): fluoride is mixed with ammonia, hydrogen, methane, ethylene, or carbon monoxide. ◦ ◦ ◦ H298 = f H 298,P − f H 298,R = E0 Understanding the nature of the structure–property ◦ relationship is of fundamental importance for further +EZPE + HT + nRT (2) 1491 1492 Hongchen Du where H ◦ and H ◦ are the sum of the (-0.195) f 298,P f 298,R (-0.403) heats of formation of the products and reactants, respec- 1.49 2.33 tively; E0 is the difference between the total energies (0.604) of the products and the reactants at 0 K; E is the ZPE (-0.403) difference between the zero-point vibrational energy of . 1 45 H 0 1 45 the products and the reactants; T is the difference (1.139) between the thermal correction from 0 K to 298 K of 1.35 the products and the reactants, n is the change in the (-0.167) quantity of gaseous substances, which is −1 here. In the (-0.408) reactions mentioned above, the experimental heats of Figure 2. Structural parameters of the adduct obtained at 17 formation of all reactants (BF3,NF3) are known, the the level of B3LYP/CC-PVTZ (bond lengths are in Å, Mul- heats of formation of the adduct can then be obtained liken charges(in brackets) in e). ◦ with the calculated H298. To find the possible molecular packings in crystal which is consistent with the conclusion drawn from phase, empirical Dreiding force field and polymorph John et al.8 the calculated infrared spectrum was 18 module in MS were used. Since most crystals belong then compared with the experimental results.25 Bailey to seven space groups (P21/c, P-1, P212121, P21, Pbca, claimed that the fundamental frequencies are located 19–22 C2/c, and Pna21) on the basis of statistical data, on 908 cm−1, which is also similar to the calculated the global search was confined to these groups only. result (898 cm−1), which indicates that the method By analyzing the simulation trajectory of molecular B3LYP/CC-PVTZ is suitable for this study. packing within seven space groups, the structures were arranged in their ascending energies for each group and the one having the lowest energy was selected 3.2 Molecular structure as the most possible packing with the corresponding space group. For solids, the local density approxima- Figure 2 shows the molecular structure of the adduct tion (LDA) and the generalized gradient approximation under B3LYP/CC-PVTZ level. It can be seen that in (GGA) are the most often used functionals. The GGA unit of NF3, the bond length of N-F are all similar, but developed by Perdew, Burke, and Ernzerhof (PBE) is N1-F3 is slightly larger, which denotes that N1-F3 is considered to be the standard for solids.23 weaker. The bond length of B5-F3 is 2.33 Å, thus the These possible crystal structures were then refined bond is very weak. In addition, the negative charges with the DFT GGA-RPBE method and CASTEP on F(3) (−0.195e) is also similar to F(2) and F(4), module.24 implying the same properties between F(3) and other F atoms. The total charges of NF3 and BF3 are nearly zero (0.075e and −0.075e); therefore, the adduct exists in 3. Results and Discussion basically the complex form NF3·BF3 but not ionic form. The molecular electrostatic potential (MEP) is used 3.1 The choice of method commonly in analyzing molecular reactivity and is very useful since it provides information about local polar- To verify the accuracy, the B3LYP/CC-PVTZ method ity due to the charge density distribution. After hav- was performed to optimize nitrogen trifluoride first. We ing chosen some sort of region to be visualized, a found that the point group at this level is C3V (figure 1), colour-coding convention is chosen to depict the MEP. 890 200 150 ty/ km/mol 100 i Intens 50 0 C3V 0 500 1000 1500 2000 Frequency / cm-1 Figure 1. The calculated structure and infrared spectrum of NF3 under B3LYP/CC-PVTZ level. Structures of NF3 and its Adduct with BF3 1493 Figure 3. Molecular electrostatic potential (MEP) surface mapped onto 0.001 electron/bohr3 contour of the electronic density for the title compound calculated at the B3LYP/-cc-PVTZ level Colour range: from red (negative) to blue (positive). Figure 3 illustrates the MEP for the 0.001 electron/ properties, such as polymorphism and growth morphol- 3 bohr isosurface of electron density at the B3LYP/CC- ogy. Elatt can be calculated from the energy difference PVTZ level for the NF3·BF3. Red denotes the most between the crystal (Ecrystal) and the isolated molecules negative potential and blue denotes the most positive (Emolcule), that is potential. On inspection of the MEP for the title com- pound, the negative potentials appear to be distributed Elatt = Ecrystal − ZEmolecule (3) mostly on the fluorine atoms, and the positive ranges characterize at the centre of the skeleton, mainly on the where, Z is the number of molecules in unit cell and nitrogen and boron atoms. is equal to 4 here. Elatt is therefore the energy required for vaporizing a crystal and represents the strength of cohesion or interaction between molecules in the 3.3 Gas-phase heats of formation solid state. A negative value of Elatt indicates an attrac- tive intermolecular interaction in a crystal. The lattice Heat of formation is usually taken as the indicator of energy of NF ·BF obtained at DFT GGA/RPBE level the ‘energy content’ of a compound. It is very impor- 3 3 is −20.19 kJ·mol−1. tant to predict the heat of formation accurately. The gas- E was further used to evaluate the enthalpy of phase heat of formation ( H ◦ ) has been estimated latt f gas sublimation (H ) using the following equation:26 using the above isodesmic reaction.