СПЕКТРОСКОПИЧЕСКИЕ И ТЕРМОХИМИЧЕСКИЕ СВОЙСТВА СОЕДИНЕНИЙ АКТИНОИДОВ ИЗ ПЕРВЫХ ПРИНЦИПОВ: МОЛЕКУЛЫ МОНОКСИДОВ ТОРИЯ И АМЕРИЦИЯ
Аннотация
Разработан релятивистский вариант составного (композиционного) ab initio вычисления спектроскопических и термохимических свойств молекул, предназначенный для высокоточного описания характеристик соединений актиноидов (An). В его основе лежит сочетание результатов расчетов квантово-химическими методами, очень полно учитывающими электронную корреляцию, например, CCSDT(Q), но принимающими во внимание только скалярные релятивистские эффекты, с результатами применения более строгих четырехкомпонентных релятивистских методов с гамильтонианом Дирака–Кулона. Очень высокая точность предложенного подхода продемонстрирована на примере молекул моноксидов тория и америция. Погрешность теоретических величин равновесного межъядерного расстояния re, частоты колебания ωe и энергии атомизации D0 молекулы ThO не превысила 0,001 Å, 2,5 см–1 и 0,5 ккал/моль соответственно. Первые потенциалы ионизации молекулы AmO и атома Am, вычисленные составным методом, отличаются от экспериментальных величин всего лишь на 0,03 эВ и 1 см–1 соответственно. Предложенный подход позволил впервые с высокой точностью определить параметры re, ωe и D0 молекулы AmO и молекулярного иона AmO+, а также второй и третий потенциалы ионизации атома америция. Результаты расчетов свидетельствуют о небольшом актиноидном сжатии связей An–O при переходе слева направо в ряду молекул ThO → UO → AmO: связь Am–O на 0,0073 Å короче, чем связь Th–O. Обнаружено, что величина re(An–O) линейно зависит от порядкового номера актиноида в периодической системе элементов. Результаты расчетов могут быть использованы в качестве эталона при параметризации и калибровке функционалов электронной плотности, применяемых при изучении An-содержащих молекул методами DFT.
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