Symmetry breaking and structural distortions in charged XH₄(X = C, Si, Ge, Sn and Pb) molecules

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We have investigated the ground state structures of neutral and charged XH₄(X=C, Si, Ge, Sn and Pb) molecules using the first-principles electronic structure methods. The structure of positively charged molecules for X = Si, Ge, Sn and Pb is characterized by a severe distortion from tetrahedral structure and an unusual H-H bond while the negatively charged molecules get distorted by pushing two hydrogen atoms away from each other. However, CH₄⁺ and CH₄⁻ are exceptions to this behavior. We provide an insight into the symmetry breaking mechanism and unusual H-H bonding using simple electrostatic arguments based on the unequal charge distribution on H atoms. Those charged molecules having unequal charge distribution on H atoms get distorted due to different electrostatic forces between the atoms. We show that the directionality and occupation of the highest occupied molecular orbital(HOMO) play an important role in creating charge asymmetry in these molecules.