Molecular and Ionic Clusters of Rubidium Fluoride: Theoretical Study of Structure and Vibrational Spectra
Ismail Abubakari1, 2, *, Tatiana Pogrebnaya1, 2, Alexander Pogrebnoi1, 2
1The Nelson Mandela African Institution of Science and Technology (NM – AIST), Arusha, Tanzania
2Dept. of Materials, Energy Science and Engineering, The NM - AIST, Arusha, Tanzania
To cite this article:
Ismail Abubakari, Tatiana Pogrebnaya, Alexander Pogrebnoi. Molecular and Ionic Clusters of Rubidium Fluoride: Theoretical Study of Structure and Vibrational Spectra.International Journal of Computational and Theoretical Chemistry.Vol.3, No. 5, 2015, pp. 34-44. doi: 10.11648/j.ijctc.20150305.11
Abstract: In this study, the geometrical structure and vibrational spectra of the trimer molecule Rb3F3 and ionic clusters Rb2F+, RbF2–, Rb3F2+, and Rb2F3– were studied by density functional theory (DFT) with hybrid functional B3P86 and Møller–Plesset perturbation theory of second order (MP2). The effective core potential with Def2–TZVP (6s4p3d) basis set for rubidium atom and aug–cc–pVTZ (5s4p3d2f) basis set for fluorine atom were used. The triatomic ions have a linear equilibrium geometric structure of D∞h symmetry, whereas for pentaatomic ions Rb3F2+, Rb2F3– and trimer molecule Rb3F3 different isomers have been revealed. For the ions Rb3F2+, Rb2F3– three isomers were confirmed to be equilibrium; the linear (D∞h), the planar cyclic (C2v) and the bipyramidal (D3h) while for trimer Rb3F3, two isomers were found; the hexagonal (D3h) and the "butterfly-shaped" (C2v) configuration.
Keywords: Geometrical Structure, Vibrational Spetra, Ionic Clusters, Hybrid Functional, Density Functional Theory,
Møller–Plesset Perturbation Theory, Effective Core Potential, Isomers, Basis Set
Rubidium halides RbX (X is a halogen) may form molecular and ionic clusters in saturated vapours. These species are characterized by different geometric structures, vibrational spectra, and thermodynamic properties depending on the number of atoms comprising the species . Other properties of clusters like electronic, optical, magnetic, and structural are strongly depend on their size and composition [2,3] thus the possibility that materials with desired properties can be made is accustomed by changing the magnitude and structure of the cluster aggregates . Extensive studies of the properties of alkali halide cluster ions had been done previously using different methods [5–15]. High temperature mass spectrometry is well known as a successful method for investigation of ionic clusters in gaseous phase [16–22]. Some ions M2X+, MX2–, M3X2+ and M2X3– were already recorded in vapours over alkali metal halides [16,17,20,22,23]. The cluster ions and neutral molecules over rubidium halides have been detected and studied by mass spectrometric technique [20,23,24]. Theoretical quantum chemical methods have been proved to be useful tools in attaining the characteristics of ions and molecules . For the rubidium halides, quantum chemical methods have been used to study the structure and properties of some ionic clusters over rubidium iodide, Rb2I+, RbI2–, Rb3I2+ and Rb2I3–  and rubidium chloride, Rb2Cl+, RbCl2–, Rb3Cl2+, and Rb2Cl3– . This study aims the theoretical investigation of neutral and ionic clusters of rubidium fluoride.
The calculations were performed using the density functional theory (DFT) with hybrid functional the Becke–Perdew correlation B3P86 [26–29], and second order Møller–Plesset perturbation theory (MP2) implemented into PC GAMESS (General Atomic and Molecular Electronic Structure System) program  and Firefly version 8.1.0 . The effective core potentials with Def2–TZVP (6s4p3d) basis set for Rb atom [32,33] and aug–cc–pVTZ basis set (5s4p3d2f) for F atom [32,34] were used. The bases set were taken from the EMSL (The Environmental Molecular Sciences Laboratory, GAMESS US), Basis Set Exchange version 1.2.2 library [35,36]. For visualisation of the geometrical structure, specification of parameters, and assignment of vibrational modes in infrared spectra the Chemcraft software  and MacMolPlt program  were applied.
The thermodynamic functions were calculated within the rigid rotator-harmonic oscillator approximation using the Openthermo software . The values of energies ∆rE and enthalpies ∆rH°(0) of the reactions were computed as follows:
∆rE = ∑Ei prod – ∑Ei reactant (1)
ΔrH°(0) = ∆rE + ∆rε (2)
∆rε = 1/2hc(∑ωi prod – ∑ωi reactant) (3)
where ∑Ei prod, ∑Ei reactant are the sums of the total energies of the products and reactants respectively, ∆rε is the zero point vibration energy correction, ∑ωi prod and ∑ωi reactant are the sums of the vibration frequencies of the products and reactants respectively.
3. Results and Discussion
3.1. Diatomic RbF and Dimer Rb2F2 Molecules
To come up with the more appropriate methods to be used for calculations, three different DFT hybrid functionals, B3LYP5, B3P86 and B3PW91 and Møller–Plesset perturbation theory (MP2) have been used in calculations of properties of the diatomic RbF and dimer Rb2F2 molecules. Then the calculated properties have been compared with the experimental data to choose the best performing method to be used in complex ionic and molecular cluster of rubidium fluoride.
The calculated equilibrium geometrical parameters, normal vibrational frequencies, ionization energy and dipole moments of the RbF molecule are shown in Table 1.
As is seen in Table 1, the internuclear distance (Re) of the diatomic molecule RbF computed using all methods are highly overrated, longer than the reference data by ~0.08–0.09 Å; the values of frequency (ωe) are underrated by 14–27 cm–1 (~5–7%) if compared with the experimental data; the values of the dipole moment (μe) are overrated by ~0.6–1.5 D.
The values of the ionization energies, adiabatic IEad and vertical IEvert, were obtained as the energy difference of the RbF+ ion and neutral molecule; for the adiabatic IEad internuclear separation Re(Rb-F) was optimized both for neutral and ionic species, while the vertical IEvert was calculated using the optimized value of Re(Rb-F) in neutral molecule only and accepted the same for the ion.
|ωe,(Σu+)||349||357||353||362||376 [41, 42]|
Notes: Here and hereafter, Re is the equilibrium internuclear distance (Å), E is the total electron energy (au), ωe is the fundamental frequency (cm–1), μe is the dipole moment (D), IEvert and IEad are the ionization energies, vertical and adiabatic, respectively (eV).
| || |
|Figure 1. Equilibrium geometric structures: (a) Rb2F2; (b) Rb2F+; (c) RbF2–.|
The IEad by MP2 method was not found in this study because the optimization procedure by MP2 was not implemented for the species with multiplicity more than 1 in the software [30,31]. The reference data on the IE of RbF to the best of our knowledge are not available. The theoretical values on IEvert as well as IEad found by all four methods are in a good agreement with each other, respectively, IEvert being by ~0.5 eV higher than IEad.
For the dimeric molecule Rb2F2 the structure was confirmed to be planar of D2h symmetry (Fig. 1a); the results are tabulated in Table 2. The geometrical parameters and vibrational frequencies calculated by different methods are in accordance with each other and literature data [44,45] as well.
The IR spectrum of Rb2F2 calculated by the MP2 method is shown in Fig. 2. Three peaks observed are assigned to asymmetrical stretching Rb-F modes at 248 cm–1 and 289 cm–1, and bending out of plane vibration at 91 cm–1. Among these three IR active modes, two have highest intensity and correspond to those which had been measured experimentally in IR spectrum in Ar matrix .
The dissociation reaction Rb2F2 → 2RbF was considered and energy and enthalpy of the reaction was calculated using Eqs. (1)-(3). The values of ∆rH°(0) found are in fair agreement with the reference data obtained from IVTANTHERMO database , the best result by the MP2 being overrated by 2 kJ×mol-1.
In conclusion of this section we can state that the data obtained by MP2 and DFT/B3P86 for the diatomic RbF and dimer Rb2F2 molecules, agree better with the available reference experimental data as compared with other DFT hybrid functionals considered. Therefore DFT/B3P86 and MP2 methods were chosen for further computations of other ionic and molecular clusters.
|ω5 (B2u)||232||238||233||248||230 |
|ω6 (B3u)||268||275||269||289||266 |
Notes: a The MP2 calculation. Here and hereafter, αe is the valence angle (deg). ∆rE and ∆rH°(0) are the energy and enthalpy of the dissociation reaction (kJ×mol-1). The reducible vibration representation reduces to Γ = 2Ag + B1g + B1u + B2u + B3u.
Note: Here and hereafter, Ii are the infrared intensities (D2×amu–1×Å–2). The reducible vibration representation reduces to Γ = Σg+ + Σu+ + Pu
3.2. Triatomic Rb2F+ and RbF2–Ions
Two structures were considered, linear of D∞h symmetry and V-shaped of C2V symmetry; the latter converged to linear during optimization and only linear structure was proved to be equilibrium (Figs. 1b, 1c). The calculated characteristics of the triatomic ions, Rb2F+ and RbF2– are summarized in Table 3.
The values obtained by the two methods are generally in agreement with each other. By comparing positive and negative triatomic ions, the internuclear distance in negative ion RbF2– is longer by approximately 0.06 Å due to an extra negative charge; in accordance to this the asymmetrical valence vibrational frequency ω2 is higher for positive ion than for negative. The deformational frequency ω3 of the negative ion is almost twice less compared the positive ion, that indicates the floppy deformational potential of the RbF2– ion. When compared with the similar ions for CsF , it may be noted the linear structure for Cs2F+ alike Rb2F+ and floppy V-shaped structure of CsF2- with the angle of about 150°. In the spectra of these ions for CsF a correspondence between frequencies of the positive and negative triatomic ions is similar to that of the ions for RbF.
3.3. Pentaatomic Rb3F2+ and Rb2F3–Ions
A number of possible geometrical configurations were considered for the pentaatomic ions Rb3F2+ and Rb2F3–, linear of D∞h symmetry, V-shaped of C2v symmetry, planar cyclic of C2v symmetry, and bipyramidal of D3h symmetry. Among them three structures were confirmed to be equilibrium, linear (D∞h), planar cyclic (C2v), and bipyramidal (D3h), they are shown in Fig. 3. The geometrical parameters and vibrational spectra are recorded in Tables 4-6.
|ω1 (Σg+)||80||84||ω1 (Σg+)||90||96|
|ω2 (Σg+)||362||374||ω2 (Σg+)||284||292|
|ω3 (Σu+)||139||147||ω3 (Σu+)||278||285|
|ω4 (Σu+)||350||367||ω4 (Σu+)||297||321|
|ω5 (Pg)||62||64||ω5 (Pg)||21||27|
|ω6 (Pu)||(2)||3||ω6 (Pu)||17||19|
|ω7 (Pu)||71||77||ω7 (Pu)||52||61|
Note: The reducible vibration representation for both positive and negative ions reduces to Γ = 2Σg+ + 2Σu+ + Pg + 2Pu
The linear isomer is specified with two non-equivalent internuclear distances, terminal Re1 and bridged Re2. For both positive and negative ions, the terminal distance is shorter than bridged one by ~0.10-0.15 Å. The linear structure was proved to be equilibrium by the absence of imaginary frequencies for negative ion and for positive according to MP2 calculations. At the same time in the DFT calculation of linear Rb3F2+ ion, the value of ω6 was imaginary. Nevertheless during the optimization procedure the bent structure of Rb3F2+ converged into slightly nonlinear without energy gain compared to linear, hence the linear configuration was accepted for this ion with the estimated value of ω6 » 2 cm-1.
The existence of very low frequencies in the vibrational spectra of the linear pentaatomic isomers implies the floppy nonrigid structure of these species with shallow bending potential. For such species the entropy is assumed to be rather high and, based on the thermodynamic approach, the linear isomers are expected to prevail in saturated vapour compared to others. The IR spectra of the Rb3F2+ and Rb2F3– ions (D¥h) are shown in Fig. 4. For both ions four modes are active in IR spectra, and for the negative ion all of them are observed. For positive ion only two peaks at ω4 = 367 cm–1 and ω7 = 77 cm–1 are seen, while remaining two modes ω3 and ω6 are not displayed due to very low intensities.
The properties for planar cyclic structure are given in Table 5. There are three non–equivalent internuclear distance Re1, Re2 and Re3 and two valence angles αe and βe (Figs. 3c, 3d) to specify the geometric configuration. The equilibrium internuclear distances of the negative ion are slightly greater than those of positive ion. Regarding the vibrational spectra of Rb3F2+ and Rb2F3–, a similarity is seen among the respective frequencies and IR intensities as well.
Isomerization reactions from linear into cyclic configurations were considered:
Rb3F2+(D∞h) → Rb3F2+(C2v) (4)
Rb2F3–(D∞h) → Rb2F3–(C2v) (5)
The energy of each reaction is the relative energy ∆Eiso of the cyclic isomer with respect to linear one. The values of ∆Eiso (Table 5) obtained by DFT and MP2 methods are slightly different; the DFT overrates a bit the energy of the cyclic isomer compared to MP2 method. Relying on the latter results, the energy of the cyclic isomer, is higher by 5.0 kJ×mol-1 for the Rb3F2+ ion and lower by 2.5 kJ×mol-1 for Rb2F3– compared to linear isomer. Hence the cyclic isomer of both ions is close by energy to the linear one.
All in all most of the properties of the cyclic isomers of positive and negative pentaatomic ions look alike.
Results for bipyramidal isomers are shown in Table 6. Only one equilibrium internuclear distance Re(Rb–F) and one valence angle are needed to specify the geometric configuration (Figs. 3e, 3f). The internuclear distances of positive and negative ion are almost the same within the same method of calculation. Valence angle at the vertex of the bipyramidal is obtuse for Rb3F2+ and acute for Rb2F3–. This tells us that the positively charged bipyramidal ion is slightly flattened compared to negatively charged one.
For the bipyramidal structure, in the vibrational spectra the lowest frequencies are about 70 cm–1 (Rb3F2+) and ~120 cm–1 (Rb2F3–). The absence of low frequencies in spectra and the bipyramidal shape itself indicate the rigidity and compactness of these isomers.
|ω1 (A1)||295 (1.51)||308 (1.64)||ω1 (A1)||271 (2.20)||280 (2.11)|
|ω2 (A1)||73 (0.02)||77 (0.04)||ω2 (A1)||78 (0.07)||82 (0.41)|
|ω3 (A1)||91 (0.01)||99 (0.02)||ω3 (A1)||79 (0.36)||82 (0.13)|
|ω4 (A1)||263 (2.14)||271 (2.05)||ω4 (A1)||249 (1.85)||258 (1.95)|
|ω5 (B1)||105 (1.32)||110 (1.34)||ω5 (B1)||123 (1.07)||145 (1.14)|
|ω6 (B1)||39 (0.19)||38 (0.22)||ω6 (B1)||25 (0.12)||26 (0.33)|
|ω7 (B2)||311 (2.36)||322 (2.34)||ω7 (B2)||291 (1.57)||299 (1.71)|
|ω8 (B2)||134 (0.86)||156 (0.90)||ω8 (B2)||158 (1.45)||175 (1.27)|
|ω9 (B2)||40 (0.17)||42 (0.14)||ω9 (B2)||53 (2.11)||52 (2.17)|
Note: ∆Eiso = E(C2v) – E(D∞h) is the relative energy of planar cyclic isomer regarding the linear one (kJ×mol-1). The values given in parentheses near the frequencies are infrared intensities (D2×amu-1×Å-2). The reducible vibration representation reduces into Γ = 4A1 + 2B1 + 3B2
For the isomerization reactions
Rb3F2+(D∞h) → Rb3F2+(D3h) (6)
Rb2F3–(D∞h) → Rb2F3–(D3h) (7)
the energies ∆Eiso = E(D3h)–E(D∞h) are given in Table 6. The values of ∆Eiso for positive and negative ions are negative, except Rb3F2+ by DFT. The negative values of ∆Eiso indicate that the bipyramidal isomers are energetically more stable than linear isomers.
Thus three isomers for pentaatomic ions were revealed to exist. In order to come up with the conclusion of the most abundant isomer in saturated vapour, the relative concentrations were calculated using the following relation:
∆rH°(0) = –RTln(piso/p) + T∆rΦ°(T) (8)
where piso/p is the ratio of the pressure of the cyclic or bipyramidal isomer to that of linear one, R is the gas constant, T is the absolute temperature, ∆rH°(0) and ∆rΦ°(T) are the enthalpy and change in the reduced Gibbs free energy of the isomerization reactions.
The reduced Gibbs free energy was found by the equation
Φ°(T) = (9)
The thermodynamic functions, enthalpy increments H°(T)-H°(0), reduced Gibbs energies Φ° (J×mol-1×K-1), and entropies S°(T) were calculated for the temperature range 298-2000 K and gathered in Appendix. The required geometrical parameters and vibrational frequencies were used as obtained by MP2 method. The values of ∆rH°(0) for the isomerisation reactions (4)-(7) were found by Eqs. (1)-(3).
|ω4 (Eʹ)||219||235||ω4 (Eʹ)||216||229|
|ω5 (Eʹ)||67||70||ω5 (Eʹ)||117||119|
|ω6 (Eʹʹ)||157||176||ω6 (Eʹʹ)||142||158|
Note: ∆Eiso = E(D3h)–E(D∞h) is the relative energy of bipyramidal isomer regarding the linear one (kJ×mol-1). The reducible vibration representation reduces into Γ = .
|Figure 4. Theoretical IR spectra of pentaatomic ions (D∞h) calculated by MP2 method: (a) Rb3F2+; (b) Rb2F3–.|
The values of piso/p were calculated for the temperature range between 700-1800 K; the plots for positive and negative ions are shown in Figs. 5a and 5b, respectively. As is seen the ratios piso/p are much less than 1 for all four cases. For the positive ion the amount of both cyclic and bipyramidal species is negligibly small and for the negative one it does not exceed 16% at 700 K and decreasing with temperature increase. Worth to note that the bipyramidal isomers are much less abundant in equilibrium vapour compared to linear despite they possess lower energy. It may be explained by prevailing of the competing entropy factor as it was predicted above. The existence of similar three isomeric forms was discussed previously regarding the pentaatomic ions over CsF  and RbI  where the linear (or close to linear) isomers were proved to be predominant in equilibrium vapours. Therefore the entropy factor is significant to conclude which isomer is predominant in equilibrium vapour.
3.4. Trimer Rb3F3 Molecule
Three possible geometrical configurations were considered for trimer molecule, Rb3F3: linear of D∞h symmetry, hexagonal of D3h symmetry and butterfly-shaped of C2v symmetry. The linear structure appeared to be non–stable as imaginary frequencies were revealed. Two other configurations were confirmed to be equilibrium (Fig. 6). The obtained geometric parameters and vibrational frequencies for the hexagonal and butterfly-shaped isomers are shown in Table 7. For the former, only one internuclear distance Re(Rb–F) and one valence angle are required to describe the structure while for the latter, four internuclear distances and two valence angles are needed. The energy ∆Eiso of the isomerization reaction
Rb3F3(D3h) → Rb3F3(C2v) (10)
|Figure 5. Temperature dependence of relative abundance of cyclic and bipyramidal isomers regarding linear: (a) Rb3F2+; (b) Rb2F3–.|
|Property||Rb3F3 (D3h)||Property||Rb3F3 (C2v)|
|αe (Rb–F–Rb)||128.3||129.1||αe(Rb2 –F5–Rb3)||99.7||99.5|
|βe(F–Rb–F)||111.7||110.9||βe(F4 – Rb1–F6)||89.4||88.1|
|178||182||ω1 (A1)||286 (3.56)||295 (3.62)|
|284||303||ω2 (A1)||118 (0.65)||140 (0.77)|
|91||92||ω3 (A1)||111 (0.13)||117 (0.08)|
|82||84||ω4 (A1)||65 (0.09)||72 (0.04)|
|ω5 (Eʹ)||313||328||ω5 (A2)||215||226|
|ω6 (Eʹ)||187||193||ω6 (A2)||49||50|
|ω7 (Eʹ)||40||38||ω7 (B1)||97 (2.00)||103 (2.02)|
|ω8 (Eʹʹ)||41||42||ω8 (B1)||15 (0.21)||24 (0.22)|
|I4||2.38||2.46||ω9 (B2)||312 (2.33)||329 (2.50)|
|I5||6.71||6.84||ω10 (B2)||269 (0.42)||280 (0.17)|
|I6||2.42||2.38||ω11 (B2)||220 (2.42)||237 (2.54)|
|I7||0.64||0.70||ω12 (B2)||79 (0.21)||85 (0.22)|
Notes: ∆Eiso = E(C2v)–E(D3h) is the relative energy of the butterfly-shaped isomer regarding the hexagonal one (kJ×mol-1). The reducible vibration representations for Rb3F3 of D3h and C2v symmetry reduce as follows: Γ = and Γ = 4A1 + 2A2 + 2B1 + 4B2, respectively. For the C2v butterfly-shaped isomer, the values given in parentheses near the frequencies are infrared intensities (D2amu-1Å-2).
|Figure 8. Theoretical IR spectra of the trimer Rb3F3 molecule calculated by MP2 method: (a) hexagonal isomer (D3h); (b) butterfly-shaped isomer (C2v).|
was determined. As compared to hexagonal, the butterfly-shaped configuration has a bit higher energy, by 2.3 kJ×mol-1, according to DFT/B3P86 method, while it has a bit lower energy, by 3.8 kJ×mol-1, according to MP2 method; that is these two isomers are close by energy to each other.
The relative concentrations of isomers have been determined similarly as it is described above in section 3.3. The ratio of p(C2v)/p(D3h) versus temperature is shown in Fig. 7. As is seen the ratio is close to one in a broad temperature range which means that two isomers compete with each other being in comparable amount. At low temperatures C2v isomer show high abundancce but its abundance decreases with temperature increase hence D3h isomer seems to dominate at elevated temperatures. Worth to mention that similar isomeric structures were revealed for the Cs3F3 molecule : one hexagonal D3h and the other of a "butterfly-shaped" (Cs); the lower symmetry of the second of Cs3F3 compared to Rb3F3 apparently due to the bigger size and higher polarizability of the caesium atom.
The IR spectra of the trimer Rb3F3 isomers are presented in Fig. 8. For the hexagonal isomer, only four modes are active in IR spectrum, and all of them are seen. In the spectrum of C2v isomer all modes, except two, ω5 and ω6 of A2 symmetry, have nonzero intensities but not all of them may be observed due to low intensities. For both isomers, the frequencies at ~200 cm-1 and above correspond to the stretching asymmetric vibrations Rb–F, the frequencies below 100 cm-1 relate to bending vibration modes. The most intensive bands are assigned to the stretching Rb–F modes, 328 cm-1 and 295 cm-1 for the D3h and C2v isomers, respectively.
A number of molecular and ionic clusters, including the trimer Rb3F3 molecule, triatomic Rb2F+ and RbF2- and pentaatomic Rb3F2+ and Rb2F3- ions, have been studied by DFT/B3P86 and MP2 methods. These methods were accepted for calculations of geometrical parameters and vibrational frequencies of the clusters because they provided a better agreement with the available reference data for diatomic RbF and dimer Rb2F2 molecules. Alternative configurations have been considered for the molecular and ionic clusters. For the triatomic ions the linear structure (D∞h) was confirmed to be equilibrium. The existence of isomers was proved for the pentaatomic ions and trimer molecule. The three isomers of comparable energy were revealed for Rb3F2+ and Rb2F3- ions: linear (D∞h), planar cyclic (C2v), and bipyramidal (D3h), the linear one being the most abundant in the equilibrium vapour compared to others. Two isomeric forms of the trimer molecule Rb3F3 were figured out: planar hexagonal (D3h) and butterfly-shaped (C2v); they were shown to have almost equal energy and comparable relative abundance in saturated vapour.
All authors participate well in all steps including computation, data analysis and manuscript preparation towards production of this work.
The authors are thankful to the Tanzania Commission for Science and Technology (COSTECH) and The Nelson Mandela African Institution of Science and Technology (NM–AIST) for support and sponsorship of this work.
The thermodynamic functions of ionic and molecular clusters, triatomic ions Rb2F+ and RbF2–, pentaatomic ions Rb3F2+ and Rb2F3–, and trimer Rb3F3 in gas phase are given in Tables A1–A6. The values of molar heat capacity cp°, entropy S°, Gibbs reduced free energy Φ° are given in J×mol-1×K-1, and enthalpy increment H°(T)–H°(0) is in kJ×mol-1, absolute temperature T in K. The thermodynamic functions of the most abundant isomers only are given for the species existing in different isomeric forms.