Rovibrational Interaction and Vibrational Constants of the Symmetric Top Molecule 14NF3

Several accurate experimental values of the α C and α B rotation-vibration interaction parameters and ω i, x ij, and g ij vibrational constants have been extracted from the most recent high-resolution Fourier transform infrared, millimeter wave, and centimeter wave investigations in the spectra of the oblate symmetric top molecule 14NF3. The band-centres used are those of the four fundamental, the overtones, the combination, and hot bands identified in the region between 400 cm−1 and 2000 cm−1. Comparison of our constants with the ones measured previously, by infrared spectroscopy at low resolution, reveals orders of magnitude higher accuracy of the new values. The agreement between our values and those determined by ab initio calculations employing the TZ2Pf basis is excellent.


Introduction
Nitrogen trifluoride NF 3 is a pyramidal symmetric top molecule belonging to C 3v point group. It has four vibrational normal modes: (i) two totally symmetric ( 1 type): ] 1 and ] 2 ; (ii) two doubly degenerate ( type): ] 3 and ] 4 . Today, NF 3 is employed in the cleaning of the PECVD chambers in the high volume production of liquid crystal displays and silicon-based thin film solar cells. It has been considered as an environmentally preferable substitute for sulphur hexafluoride or perfluorocarbons such as hexafluoroethane [1]. But, recent publications [2][3][4][5][6][7] reported that NF 3 is 17,000 times more potent a greenhouse gas than CO 2 and recommended to include it in the basket of gases controlled by Kyoto protocol and to monitor its environmental levels.
Nitrogen trifluoride is also used in hydrogen fluoride and deuterium fluoride lasers, which are types of chemical lasers.
In spite of these interesting properties and important applications, the spectra of this molecule have not yet been much studied in the past, because of difficulties to obtain high quality spectra and to interpret its perturbed bands. Many of these difficulties have now been overcome through the advent of high-resolution spectrometers and the development of computers. This is within the framework of this research orientation and the Moroccan/European collaboration that several bands of NF 3 gas were recently analysed [8][9][10][11][12][13][14]. We used independent and complementary methods, Fourier transform infrared (FTIR), millimetre wave (MMW), and centimetre wave (CMW) spectroscopies. All studied IR spectra were recorded at Wuppertal (Germany) with the Bruker 120 HR interferometer, except for the region around 1032 cm −1 , which was recorded at the University of Giessen (Germany). The rotational spectra were measured in Lille (France) with a computer-controlled MMW spectrometer. The measurements in the CMW range were performed at Kiel (Germany) by means of wave-guide microwave Fourier transform spectroscopy.
The subject of this work is to extract accurate experimental rotation-vibration interaction parameters and anharmonicity constants of the potential function of 14 NF 3 from 2 The Scientific World Journal our recent studies by high-resolution spectroscopy and the latest results available in the literature.

Relationships Linking Vibrational Constants
More recently, we have established the following relationships linking band-centres, harmonic wavenumbers, and anharmonicity constants of a symmetric top molecule [15].

Fundamental Bands
is the normal wavenumber of the th mode; ] is its vibrational quantum number; refers to a totally symmetric or to a degenerate vibration; and are the anharmonicity constants; and ℓ is the vibrational angular momentum: |ℓ | = ] , ] − 2, . . . , 1, or 0 only exists for a doubly degenerate vibration.

The Ground State Constants of 14 NF 3
The experimental ground state (GS) axial rotational parameters for the symmetric top molecule 14 NF 3 employed in the present study and reported in Table 1 come from different sources.
For the 0 -dependent constant, we used in our highresolution studies the following values: In the two cases, we used the "loop method" as described in [16].
For the 0 -dependent constant, the values adopted were determined by Höhe et al. [17], Breidung et al. [18], and Cazzoli and Puzzarini [19]. Parameter Value [12,17] Value [11,18] Value [11,19] [13]. They were analysed by FTIR and MMW spectroscopies. The corresponding excited states were treated as isolated levels, and the following band-centres were determined: (] 2 ) 0 = 647.1340617 (73) cm −1 , Because of the low intensity of the 2] 2 overtone, the ] 2 = 2 state constants were determined from the 2] 2 − ] 2 hot and ] 2 fundamental bands. We point out that any explanation was offered for a severe intensity perturbation observed in the spectra of ] 2 and 2] 2 bands (see [13]). [12]. The perpendicular band ] 3 of the symmetric top 14 NF 3 was analysed by highresolution FTIR and MMW spectroscopies. The ] 3 = 1 excited state was considered vibrationally isolated, but several intravibrational interactions were included in the final fit. The band-centre obtained is (] 3 ) 0 = 907.5413300 (72) cm −1 . [11]. The ] 4 = 2 excited state of the oblate molecule 14 NF 3 , lying near 985 cm −1 , was successfully studied by high-resolution FTIR and MMW spectroscopies. Assumed to be isolated, the treatment of this level gave the  (Tables 2 and 3). b Value estimated from our FTIR spectrum which is in course of study.

Results and Discussion
In Table 2, we gather the recent measurements, at highresolution, of the centres of the all studied bands between 400 cm −1 and 2000 cm −1 and the corresponding experimental rotation-vibration interaction constants: = 0 − and = 0 − and and being the constants of the upper level.
All experimental anharmonicity constants determined in this work for the nitrogen trifluoride 14 NF 3 are given in Table 3.
The experimental centre of the 2] 2 band was estimated from the branches edge at 1292.26 cm −1 . But a precise value can be obtained combining the values of the experimental centres of ] 2 and 2] 2 −] 2 ( Table 2). From the relationships: The Scientific World Journal  It is worth noting that our values of the anharmonicity constants of 14 NF 3 are in good agreement with previous medium experimental studies (column 4 of Table 3), but significantly more accurate by at least three orders of magnitude.
One can notice the fair agreement between our values and those obtained by ab initio methods [18] employing the TZ2Pf basis.
We can also extract accurate values of the harmonic wavenumbers of the oblate symmetric top molecule 14 NF 3 . The results are gathered in Table 4.
The agreement between our values and those determined by the theoretical ab initio calculations is significantly worse. We think that it is necessary to give much more credibility to our 0 values which were deduced from experimental constants.

Conclusion
Using the recent accurate results obtained for the bands below 2000 cm −1 , of the symmetric top molecule 14 NF 3 , several rovibrational interaction and anharmonicity constants have been deduced.
Our results contribute incontestably to the experimental knowledge of the molecular potential of this molecule which helps to test and improve theoretical models.