دراسة تأثير الشحنات الموجبة (+1 ,+2) والسالبة (-1 ,-2) على الشكل الهندسي والأطياف الأهتزازية لآيزومرات البروموفولفين بإسلوب نظرية دالية الكثافة DFT

Study the Effect of the Positive (+1,+2) and Negative (-1,-2) Charges on the Geometry and Vibrational Spectra of Bromofulvene Isomers with DFT model

This study included using of Quantum mechanical calculation method (DFT method and B3LYP/CEP-121G bases set) for bromofulvene isomers with three different positions C₃ , C₄ ,C₆  as well as their positive (+1,+2) and negative (-1,-2) radical ions in order to evaluate the optimization of geometry , total energy , electron densities , spin densities , effect of the substitute on stability of each one of studied molecules and their radical ions (isodesmic reaction) , ionization energies , dipole moments , orbital energies( energies of HOMO and LUMO orbital) , HOMO-LUMO energy gaps , standard thermodynamic functions U⁰  ،H⁰  ،S⁰ ، G⁰ and A⁰, electronic transitions (UV/visible spectroscopy) , the 3N-6 fundamental vibration frequencies  along  with their  corresponding IR absorption intensities , an assignment for each fundamental mode was also determined and we find the following results:-

•  C-Br bond is the longest compared with other bonds in all states of isomer (neutral,-1,-2,+1,+2) , by comparison ,a C-Br length in radical anions is longer than its in neutral ,this may be due to repulsion between  a non-bonding electron pairs on Br with a delocalization of negative charge throughout an entire molecule (C₃ ,C₄ ,C₆ ) . Removing one or two electrons from a molecule leads to shorten C-Br length , this may be attributed to movement of non-bonding electron pairs on Br extended along C-Br bond associating with conjugated  system . The C-Br lengths are as follow :

Anion(-2) Anion(-1) Neutral Cation(+1) Cation(+2)

Adding one or two electrons to the molecule to form (-1,-2) and removing to form (+1,+2) caused in an increasing a length of R(1-2) , R(3-4) ,     R(5-6) bonds and shortening R(4-5) , R(2-3) , R(1-5) length .

•  HOMO and LUMO energies for all isomers are as follow :

Anion(-2) Anion(-1) Neutral Cation(+1) Cation(+2)

Ionization is indirectly related with HOMO energies , so its values are :

Cation (+2) Neutral  Anion(-2)

The largest , the (E_HOMO-LUMO ) for all isomers is at a neutral state .

 By comparison between isomers themselves in ( E_HOMO-LUMO) , it’s found that 3-Bromofulvene isomer has the largest value for (neutral , -1) states , and 6-Bromofulvene isomer for (+1,-2) states , then 4-Bromofulvene isomer for the (+2) state .

• The radical anions (-1,-2) of three bromofulvene isomers have the highest values of electron densities, and then neutral state , while          the lowest electron densities are for radical cations (+1,+2) to same isomer excepting two states, the first is substituted position and second is C₅ position . By comparison between isomers themselves in electron densities , it’s found that the substituted position is the most rich electron density in comparison with it in other isomers ,this may be attributed to substitute effect . 3-Bromofulvene have the largest electron density at position C₅ , and 4-Bromofulvene at C₂ , while 6-Bromofulvene at C₁ .
•  Spin densities : in radical anions , spin  resides largely at C₆ position , to a small degree at C₁ and C₄ , and even less at C₂ and C₃ . There is no spin density at other positions in radical anions . In radical cations , the spin  resides largely at C₁ and C₄ positions , to a small degree at C₃ , and even less at Br and C₆ .
• Stabilizing effect of substituents is often assessed by using isodesmic reaction . A positive (∆) indicates stabilization of a reactant by                 a substituent . Results show that the radical anions (-1,-2) are stabilizing  by Br substituent , this may be attributed to a Br withdrawing inductive effect that decrease the excess of electronic charge . But the radical cations and isomers with neutral state are instable with Br substituent because an electron deficiency , except the less stable (+2) state of 4-Bromofulvene isomer .
• Dipole moment increases as charge increases , so the (-2,+2) states have the largest value of  dipole moment . By comparison with other isomers , 3-Bromofulvene are the largest in Dipole moment for (-1,-2,+1, neutral) states , while (+2) state , it’s found that the 6-Bromofulvene is the    highest .
•  Neutral state for three isomers have the lowest entropy . By comparison between the isomers themselves , it’s found that 6-Bromofulvene is the lowest entropy in (-1,-2) states , but (neutral ,+1,+2) have the lowest entropy in 4-Bromofulvene .
•  For all isomers , the ionization potentials are :

Cation(+2)  Cation(+1)  Neutral  Anion(-1)  Anion(-2)

By comparison between the isomers themselves , it’s found that , for (neutral ,-1) states , 3-Bromofulvene have the largest value, while in the two radicals (-2,+1) ,6-Bromofulvene is the highest ionization potentials . And 4-Bromofulvene in (+2) state only.

•  Electronic transitions: for all isomers , first excited states are expected to be ( ) type because their wavelengths are longer   (411.2-761.07) nm than the second and third the excited states (263.44-310.14) nm that may be due to (  ) type because their shorter wavelength . results show that the two type of transition are shifted to longer wave length (bathochromic shift) when a molecule lose or acquire an electron and that may be result from delocalization of electrons because presence of negative and positive charge causing in increasing the s character on chromophore groups (C=C) , except first excited state transition in 3-Bromofulvene and 6-Bromofulvene in  neutral state that are shifted to shorter wavelength (Hypsochromic shift ) .
•  Asymmetric stretching vibrational frequencies of CH₂ have larger values than that of a symmetric stretching vibrational frequencies for the all states of 3-Bromofulvene and 4-Bromofulvene :

CH₂as.st.  > CH₂s.st.

By comparison between (neutral , -1 ,-2 ,+1,+2) states for the two isomers ; 3-Bromofulvene and 4-Bromofulvene , it’s found that radical cations have the highest values of symmetric and asymmetric stretching vibrational frequencies and then neutral state , while the lowest values are in radical anions . 4-Bromofulvene have larger symmetric and asymmetric stretching vibrational frequencies compared with 3-Bromofulvene in all states (neutral , -1 ,-2 ,+1,+2) may be due to the closeness of 4-Bromofulvene to a exocyclic double bond carrying methylene group and then 3-Bromofulvene , and for this type of frequencies , the radical anion (-2) have the largest absorption intensities .

•  Highest frequencies values for the four bending vibrational (scissoring , rocking ,twisting ,wagging) were to the radical cations , and  cases that the neutral states frequencies was larger than to the radical cation . 4-Bromofulvene isomer have larger frequencies than 3-Bromofulvene for the most of states . The largest absorption intensities for this type of frequencies for both 4-Bromofulvene and 3-Bromofulvene were observed at (-2,+2) radicals .
• The stretching vibrational frequency for C₅=C₆ is giving as follow :

Neutral  Cation(+1)  Cation(+2)  Anion(-1)  Anion(-2)

•  In general , what were observed  for most of  C-H and CC stretching and bending  vibrational frequencies , that the largest values were to radical cations , this may be attributed to the increasing of ( s character ) resulting from the electron deficiency