The geometry and optimization of La-MPBCP or [La-MPBCP]+3 systems
The studied La-MPBCP or [La-MPBCP]+3 systems in this work are not planar, they belong to the C3V point group. For both systems, the lanthanum atom was preferably adsorbed in the middle of the cavity. The coordination sphere about the lanthanum possesses a trigonal pyramidal geometry, with La-N bond lengths of (2.58 Å) La-MPBCP and (2.80 Å) [La-MPBCP]+3, these values were consistent with similar experimental bond data (2.86-2.68 Å) reported before. 47-48. The coordination is better in the neutral system as expected by the formation of stronger bonds and a N-La-N bond angle of 118°. The optimized structures are depicted in Figure 3. The La-MPBCP system undergoes a considerable distortion in order to keep the nitrogen donors close to the lanthanum center and this system shows a greater stiffness caused by the N-La interaction, allowing the lanthanum atom to lie closer to the plane of the MPBCP. Adsorption energy values, as well as some important parameters for both systems, are listed in Table 1.
The highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO are the main orbitals that participate in the chemical stability they are shown in Figure 4, and the corresponding energy levels are listed in Table 1. The HOMOs are predominantly localized on the MPBCP ligand, while the LUMOs are localized mainly on lanthanum atoms. The energy gap between HOMO and LUMO of the MPBCP is 4.22 eV and our calculated band gaps for La-systems show that the coordination of lanthanum atom on the ligand affect the non-conductor character of MPBCP. The [La-MPBCP]+3 system exhibits lower band gaps of 1.39 eV, indicating character de semiconductor and more excitability system. (see Table 1)
The predicted IR spectrum of MPBCP, La-MPBCP and [La-MPBCP]+3 system shows bands associated respectively to the stretching modes of N-H, C-H, La-N of group of pyrrole and benzene ring. The IR data calculated of the MPBCP exhibit bands at 1507 cm–1 and 3207 cm–1, due to the pyrrole ν(C=N) group and ν(C-H) bond of benzene. The band corresponding to the C=N group was considerably shifted to the lower frequency 122 cm–1(1360 cm–1) and 120 cm–1 (1377 cm−1) in their corresponding La-MPBCP and [La-MPBCP]+3 system indicating the involvement of the nitrogen atoms in the coordination. The presence of two bands at 1135, 323 cm–1 and 917, 306 cm–1attributed to ν(La–N) respectively. The calculated for assignments for the MPBCP, La-MPBCP and [La-MPBCP]+3 system in gas phase are collected in Table 2.
The study of natural bond orbitals shows the lone pairs from nitrogen atoms participate as donors, while the lanthanum atom as acceptor see Figure 3c and3d. Charges on La atoms are +2.30, +2.40 in La-MPBCP and [La-MPBCP]+3, respectively. Charges on N atoms in the cavity are -0.79 to -0.81. The nitrogen atoms in [La-MPBCP]+3 system has the most negative charge, it is clear that this systems is the electron-deficient La site.