Nonstandard behavior of a negative ion reaction at very low temperatures

We have studied the negative ion reaction NH2-+H_{2}-->NH_{3}+H- in the temperature range from 300 to 8 K. We observe a strongly suppressed probability for proton transfer at room temperature. With decreasing temperature, this probability increases, in accordance with a longer lifetime of an intermediate anion-neutral complex. At low temperatures, a maximum in the reaction rate coefficient is observed that suggests the presence of a very small barrier at long range or a quantum mechanical resonance feature.