Solid-state versus solution preparation of two crystal forms of [HN(CH2CH2)3NH][OOC(CH2)COOH]2. Polymorphs or hydrogen bond isomers?

The reaction of [N(CH2CH2)3N] with malonic acid [HOOC(CH2)COOH] in the molar 1:2 ratio yields two different crystal forms of the salt [HN(CH2CH2)3NH][OOC(CH2)COOH]2 depending on the prepn. technique and crystn. speed: form I, contg. malonate anions with intramol. hydrogen bonds, is obtained by solid-state co-grinding or by rapid crystn., whereas form II contg. intermol. hydrogen bonds is obtained by slow crystn. Form I and II do not interconvert, and form I undergoes an order-disorder phase transition on cooling