AIEEE 2007 Solved Paper

Among $\mathrm{F}, \mathrm{O}$ and $\mathrm{N}, \mathrm{F}$ is most electronegative so $\mathrm{F}$ pulls bond pair of electron in $\mathrm{F}$ - $\mathrm{H}$ towards itself and develops highly positive charge on $\mathrm{H}$ atom.This highly positive charged $\mathrm{H}$ atom creates strongest hydrogen bonding by taking lone pair of electron form electronegative atom $\mathrm{F}/\mathrm{N}/\mathrm{O}$. Hence among the given options $\mathrm{F}-\mathrm{H}\cdots\cdots\mathrm{F}$ is the strongest bond.Note : $\mathrm{F}-\mathrm{H}\cdots\cdots\mathrm{N}$ has strongest hydrogen bonding among $\mathrm{F}-\mathrm{H}\cdots\cdots\mathrm{F}, \mathrm{F}-\mathrm{H}\cdots\cdots\mathrm{O}$ and $\mathrm{F}-$ $\mathrm{H}$...... $\mathrm{N}$ because $\mathrm{N}$ is least electronegative among $\mathrm{F}, \mathrm{O}$ and $\mathrm{N}$ and can easily donate lone pair of electron.