Unconstraint, abandon, spontaneity can all denote the free and uninhibited expression of one’s thoughts or feelings or the quality of mood or style resulting from a free yielding to impulse.
Unconstraint is the most general term and may be used in place of either of the others, though it is less positive in its implication.
Abandon adds to unconstraint the implication either of entire loss of self-control or of the absence or impotence of any influence hampering free, full, or natural expression of feeling.
Spontaneity suggests an unstudied naturalness and agreeable freshness of expression or manner; sometimes it connotes lack of deliberation and obedience to the impulse of the moment.